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Bryozoa

Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about 0.5 mm long, they have a special feeding structure called a lophophore, a "crown" of tentacles used for filter feeding. Most marine bryozoans live in tropical waters, but a few are found in oceanic trenches and polar waters. The bryozoans are classified as the marine bryozoans (Stenolaemata), freshwater bryozoans (Phylactolaemata), and mostly-marine bryozoans (Gymnolaemata), a few members of which prefer brackish water. 5,869living species are known. Originally all of the crown group Bryozoa were colonial, but as an adaptation to a mesopsammal (interstitial spaces in marine sand) life or to deep‐sea habitats, secondarily solitary forms have since evolved. Solitary species has been described in four genera; Aethozooides, Aethozoon, Franzenella and Monobryozoon). The latter having a statocyst‐like organ with a supposed excretory function.

The terms Polyzoa and Bryozoa were introduced in 1830 and 1831, respectively. Soon after it was named, another group of animals was discovered whose filtering mechanism looked similar, so it was included in Bryozoa until 1869, when the two groups were noted to be very different internally. The new group was given the name "Entoprocta", while the original Bryozoa were called "Ectoprocta". Disagreements about terminology persisted well into the 20th century, but "Bryozoa" is now the generally accepted term.

Colonies take a variety of forms, including fans, bushes and sheets. Single animals, called zooids, live throughout the colony and are not fully independent. These individuals can have unique and diverse functions. All colonies have "autozooids", which are responsible for feeding, excretion, and supplying nutrients to the colony through diverse channels. Some classes have specialist zooids like hatcheries for fertilized eggs, colonial defence structures, and root-like attachment structures. Cheilostomata is the most diverse order of bryozoan, possibly because its members have the widest range of specialist zooids. They have mineralized exoskeletons and form single-layered sheets which encrust over surfaces, and some colonies can creep very slowly by using spiny defensive zooids as legs.

Each zooid consists of a "cystid", which provides the body wall and produces the exoskeleton, and a "polypide", which holds the organs. Zooids have no special excretory organs, and autozooids' polypides are scrapped when they become overloaded with waste products; usually the body wall then grows a replacement polypide. Their gut is U-shaped, with the mouth inside the crown of tentacles and the anus outside it. Zooids of all the freshwater species are simultaneous hermaphrodites. Although those of many marine species function first as males and then as females, their colonies always contain a combination of zooids that are in their male and female stages. All species emit sperm into the water. Some also release ova into the water, while others capture sperm via their tentacles to fertilize their ova internally. In some species the larvae have large yolks, go to feed, and quickly settle on a surface. Others produce larvae that have little yolk but swim and feed for a few days before settling. After settling, all larvae undergo a radical metamorphosis that destroys and rebuilds almost all the internal tissues. Freshwater species also produce statoblasts that lie dormant until conditions are favorable, which enables a colony's lineage to survive even if severe conditions kill the mother colony.

Predators of marine bryozoans include sea slugs (nudibranchs), fish, sea urchins, pycnogonids, crustaceans, mites and starfish. Freshwater bryozoans are preyed on by snails, insects, and fish. In Thailand, many populations of one freshwater species have been wiped out by an introduced species of snail. Membranipora membranacea, a fast-growing invasive bryozoan off the northeast and northwest coasts of the US, has reduced kelp forests so much that it has affected local fish and invertebrate populations. Bryozoans have spread diseases to fish farms and fishermen. Chemicals extracted from a marine bryozoan species have been investigated for treatment of cancer and Alzheimer's disease, but analyses have not been encouraging.

Mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period, making it the last major phylum to appear in the fossil record. This has led researchers to suspect that bryozoans arose earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms. In 2021, some research suggested Protomelission, a genus known from the Cambrian period, could be an example of an early bryozoan, but later research suggested that this taxon may instead represent a dasyclad alga. Early fossils are mainly of erect forms, but encrusting forms gradually became dominant. It is uncertain whether the phylum is monophyletic. Bryozoans' evolutionary relationships to other phyla are also unclear, partly because scientists' view of the family tree of animals is mainly influenced by better-known phyla. Both morphological and molecular phylogeny analyses disagree over bryozoans' relationships with entoprocts, about whether bryozoans should be grouped with brachiopods and phoronids in Lophophorata, and whether bryozoans should be considered protostomes or deuterostomes.

Distinguishing features
Bryozoans, phoronids and brachiopods strain food out of the water by means of a lophophore, a "crown" of hollow tentacles. Bryozoans form colonies consisting of clones called zooids that are typically about 0.5 mm long. Phoronids resemble bryozoan zooids but are 2 to 20 cm long and, although they often grow in clumps, do not form colonies consisting of clones. Brachiopods, generally thought to be closely related to bryozoans and phoronids, are distinguished by having shells rather like those of bivalves. All three of these phyla have a coelom, an internal cavity lined by mesothelium. Some encrusting bryozoan colonies with mineralized exoskeletons look very like small corals. However, bryozoan colonies are founded by an ancestrula, which is round rather than shaped like a normal zooid of that species. On the other hand, the founding polyp of a coral has a shape like that of its daughter polyps, and coral zooids have no coelom or lophophore.

Entoprocts, another phylum of filter-feeders, look rather like bryozoans but their lophophore-like feeding structure has solid tentacles, their anus lies inside rather than outside the base of the "crown" and they have no coelom.

Types of zooid
All bryozoans are colonial except for one genus, Monobryozoon. Individual members of a bryozoan colony are about 0.5 mm long and are known as zooids, since they are not fully independent animals. All colonies contain feeding zooids, known as autozooids. Those of some groups also contain non-feeding heterozooids, also known as polymorphic zooids, which serve a variety of functions other than feeding; colony members are genetically identical and co-operate, rather like the organs of larger animals. What type of zooid grows where in a colony is determined by chemical signals from the colony as a whole or sometimes in response to the scent of predators or rival colonies.

The bodies of all types have two main parts. The cystid consists of the body wall and whatever type of exoskeleton is secreted by the epidermis. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. The latter is always absent in freshwater species. The body wall consists of the epidermis, basal lamina (a mat of non-cellular material), connective tissue, muscles, and the mesothelium which lines the coelom (main body cavity) – except that in one class, the mesothelium is split into two separate layers, the inner one forming a membranous sac that floats freely and contains the coelom, and the outer one attached to the body wall and enclosing the membranous sac in a pseudocoelom. The other main part of the bryozoan body, known as the polypide and situated almost entirely within the cystid, contains the nervous system, digestive system, some specialized muscles and the feeding apparatus or other specialized organs that take the place of the feeding apparatus.

Feeding zooids
The most common type of zooid is the feeding autozooid, in which the polypide bears a "crown" of hollow tentacles called a lophophore, which captures food particles from the water. In all colonies a large percentage of zooids are autozooids, and some consist entirely of autozooids, some of which also engage in reproduction.

The basic shape of the "crown" is a full circle. Among the freshwater bryozoans (Phylactolaemata) the crown appears U-shaped, but this impression is created by a deep dent in the rim of the crown, which has no gap in the fringe of tentacles. The sides of the tentacles bear fine hairs called cilia, whose beating drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by mucus, and further cilia on the inner surfaces of the tentacles move the particles towards the mouth in the center. The method used by ectoprocts is called "upstream collecting", as food particles are captured before they pass through the field of cilia that creates the feeding current. This method is also used by phoronids, brachiopods and pterobranchs.

The lophophore and mouth are mounted on a flexible tube called the "invert", which can be turned inside-out and withdrawn into the polypide, rather like the finger of a rubber glove; in this position the lophophore lies inside the invert and is folded like the spokes of an umbrella. The invert is withdrawn, sometimes within 60milliseconds, by a pair of retractor muscles that are anchored at the far end of the cystid. Sensors at the tips of the tentacles may check for signs of danger before the invert and lophophore are fully extended. Extension is driven by an increase in internal fluid pressure, which species with flexible exoskeletons produce by contracting circular muscles that lie just inside the body wall, while species with a membranous sac use circular muscles to squeeze this. Some species with rigid exoskeletons have a flexible membrane that replaces part of the exoskeleton, and transverse muscles anchored on the far side of the exoskeleton increase the fluid pressure by pulling the membrane inwards. In others there is no gap in the protective skeleton, and the transverse muscles pull on a flexible sac which is connected to the water outside by a small pore; the expansion of the sac increases the pressure inside the body and pushes the invert and lophophore out. In some species the retracted invert and lophophore are protected by an operculum ("lid"), which is closed by muscles and opened by fluid pressure. In one class, a hollow lobe called the "epistome" overhangs the mouth.

The gut is U-shaped, running from the mouth, in the center of the lophophore, down into the animal's interior and then back to the anus, which is located on the invert, outside and usually below the lophophore. A network of strands of mesothelium called "funiculi" ("little ropes") connects the mesothelium covering the gut with that lining the body wall. The wall of each strand is made of mesothelium, and surrounds a space filled with fluid, thought to be blood. A colony's zooids are connected, enabling autozooids to share food with each other and with any non-feeding heterozooids. The method of connection varies between the different classes of bryozoans, ranging from quite large gaps in the body walls to small pores through which nutrients are passed by funiculi.

There is a nerve ring round the pharynx (throat) and a ganglion that serves as a brain to one side of this. Nerves run from the ring and ganglion to the tentacles and to the rest of the body. Bryozoans have no specialized sense organs, but cilia on the tentacles act as sensors. Members of the genus Bugula grow towards the sun, and therefore must be able to detect light. In colonies of some species, signals are transmitted between zooids through nerves that pass through pores in the body walls, and coordinate activities such as feeding and the retraction of lophophores.

The solitary individuals of Monobryozoon are autozooids with pear-shaped bodies. The wider ends have up to 15 short, muscular projections by which the animals anchor themselves to sand or gravel and pull themselves through the sediments.

Avicularia and vibracula
Some authorities use the term avicularia (plural of avicularium) to refer to any type of zooid in which the lophophore is replaced by an extension that serves some protective function, while others restrict the term to those that defend the colony by snapping at invaders and small predators, killing some and biting the appendages of others. In some species the snapping zooids are mounted on a peduncle (stalk), their bird-like appearance responsible for the term – Charles Darwin described these as like "the head and beak of a vulture in miniature, seated on a neck and capable of movement". Stalked avicularia are placed upside-down on their stalks. The "lower jaws" are modified versions of the opercula that protect the retracted lophophores in autozooids of some species, and are snapped shut "like a mousetrap" by similar muscles, while the beak-shaped upper jaw is the inverted body wall. In other species the avicularia are stationary box-like zooids laid the normal way up, so that the modified operculum snaps down against the body wall. In both types the modified operculum is opened by other muscles that attach to it, or by internal muscles that raise the fluid pressure by pulling on a flexible membrane. The actions of these snapping zooids are controlled by small, highly modified polypides that are located inside the "mouth" and bear tufts of short sensory cilia. These zooids appear in various positions: some take the place of autozooids, some fit into small gaps between autozooids, and small avicularia may occur on the surfaces of other zooids.

In vibracula, regarded by some as a type of avicularia, the operculum is modified to form a long bristle that has a wide range of motion. They may function as defenses against predators and invaders, or as cleaners. In some species that form mobile colonies, vibracula around the edges are used as legs for burrowing and walking.

Structural polymorphs
Kenozooids (from the Greek 'empty') consist only of the body wall and funicular strands crossing the interior, and no polypide. The functions of these zooids include forming the stems of branching structures, acting as spacers that enable colonies to grow quickly in a new direction, strengthening the colony's branches, and elevating the colony slightly above its substrate for competitive advantages against other organisms. Some kenozooids are hypothesized to be capable of storing nutrients for the colony. Because kenozooids' function is generally structural, they are called "structural polymorphs."

Some heterozooids found in extinct trepostome bryozoans, called mesozooids, are thought to have functioned to space the feeding autozooids an appropriate distance apart. In thin sections of trepostome fossils, mesozooids can be seen in between the tubes that held autozooids; they are smaller tubes that are divided along their length by diaphragms, making them look like rows of box-like chambers sandwiched between autozooidal tubes.

Reproductive polymorphs
Gonozooids act as brood chambers for fertilized eggs. Almost all modern cyclostome bryozoans have them, but they can be hard to locate on a colony because there are so few gonozooids in one colony. The aperture in gonozooids, which is called an ooeciopore, acts as a point for larvae to exit. Some gonozooids have very complex shapes with autozooidal tubes passing through chambers within them. All larvae released from a gonozooid are clones created by division of a single egg; this is called monozygotic polyembryony, and is a reproductive strategy also used by armadillos.

Cheilostome bryozoans also brood their embryos; one of the common methods is through ovicells, capsules attached to autozooids. The autozooids possessing ovicells are normally still able to feed, however, so these are not considered heterozooids.

"Female" polymorphs are more common than "male" polymorphs, but specialized zooids that produce sperm are also known. These are called androzooids, and some are found in colonies of Odontoporella bishopi, a species that is symbiotic with hermit crabs and lives on their shells. These zooids are smaller than the others and have four short tentacles and four long tentacles, unlike the autozooids which have 15–16 tentacles. Androzooids are also found in species with mobile colonies that can crawl around. It is possible that androzooids are used to exchange sperm between colonies when two mobile colonies or bryozoan-encrusted hermit crabs happen to encounter one another.

Other polymorphs
Spinozooids are hollow, movable spines, like very slender, small tubes, present on the surface of colonies, which probably are for defense. Some species have miniature nanozooids with small single-tentacled polypides, and these may grow on other zooids or within the body walls of autozooids that have degenerated.

Colony forms and composition

Although zooids are microscopic, colonies range in size from 1 cm to over 1 m. However, the majority are under 10 cm across. The shapes of colonies vary widely, depend on the pattern of budding by which they grow, the variety of zooids present and the type and amount of skeletal material they secrete.

Some marine species are bush-like or fan-like, supported by "trunks" and "branches" formed by kenozooids, with feeding autozooids growing from these. Colonies of these types are generally unmineralized but may have exoskeletons made of chitin. Others look like small corals, producing heavy lime skeletons. Many species form colonies which consist of sheets of autozooids. These sheets may form leaves, tufts or, in the genus Thalamoporella, structures that resemble an open head of lettuce.

The most common marine form, however, is encrusting, in which a one-layer sheet of zooids spreads over a hard surface or over seaweed. Some encrusting colonies may grow to over 50 cm and contain about 2,000,000 zooids. These species generally have exoskeletons reinforced with calcium carbonate, and the openings through which the lophophores protrude are on the top or outer surface. The moss-like appearance of encrusting colonies is responsible for the phylum's name (Ancient Greek words βρύον meaning 'moss' and ζῷον  meaning 'animal'). Large colonies of encrusting species often have "chimneys", gaps in the canopy of lophophores, through which they swiftly expel water that has been sieved, and thus avoid re-filtering water that is already exhausted. They are formed by patches of non-feeding heterozooids. New chimneys appear near the edges of expanding colonies, at points where the speed of the outflow is already high, and do not change position if the water flow changes.

Some freshwater species secrete a mass of gelatinous material, up to 1 m in diameter, to which the zooids stick. Other freshwater species have plant-like shapes with "trunks" and "branches", which may stand erect or spread over the surface. A few species can creep at about 2 cm per day.

Each colony grows by asexual budding from a single zooid known as the ancestrula, which is round rather than shaped like a normal zooid. This occurs at the tips of "trunks" or "branches" in forms that have this structure. Encrusting colonies grow round their edges. In species with calcareous exoskeletons, these do not mineralize until the zooids are fully grown. Colony lifespans range from one to about 12 years, and the short-lived species pass through several generations in one season.

Species that produce defensive zooids do so only when threats have already appeared, and may do so within 48 hours. The theory of "induced defenses" suggests that production of defenses is expensive and that colonies which defend themselves too early or too heavily will have reduced growth rates and lifespans. This "last minute" approach to defense is feasible because the loss of zooids to a single attack is unlikely to be significant. Colonies of some encrusting species also produce special heterozooids to limit the expansion of other encrusting organisms, especially other bryozoans. In some cases this response is more belligerent if the opposition is smaller, which suggests that zooids on the edge of a colony can somehow sense the size of the opponent. Some species consistently prevail against certain others, but most turf wars are indecisive and the combatants soon turn to growing in uncontested areas. Bryozoans competing for territory do not use the sophisticated techniques employed by sponges or corals, possibly because the shortness of bryozoan lifespans makes heavy investment in turf wars unprofitable.

Bryozoans have contributed to carbonate sedimentation in marine life since the Ordovician period. Bryozoans take responsibility for many of the colony forms, which have evolved in different taxonomic groups and vary in sediment producing ability. The nine basic bryozoan colony-forms include: encrusting, dome-shaped, palmate, foliose, fenestrate, robust branching, delicate branching, articulated and free-living. Most of these sediments come from two distinct groups of colonies: domal, delicate branching, robust branching and palmate; and fenestrate. Fenestrate colonies generate rough particles both as sediment and components of stromatoporoids coral reefs. The delicate colonies however, create both coarse sediment and form the cores of deep-water, subphotic biogenic mounds. Nearly all post- bryozoan sediments are made up of growth forms, with the addition to free-living colonies which include significant numbers of various colonies. "In contrast to the Palaeozoic, post-Palaeozoic bryozoans generated sediment varying more widely with the size of their grains; they grow as they moved from mud, to sand, to gravel."

Taxonomy

The phylum was originally called "Polyzoa", but this name was eventually replaced by Ehrenberg's term "Bryozoa". The name "Bryozoa" was originally applied only to the animals also known as Ectoprocta (lit. 'outside-anus'), in which the anus lies outside the "crown" of tentacles. After the discovery of the Entoprocta (lit. 'inside-anus'), in which the anus lies within a "crown" of tentacles, the name "Bryozoa" was promoted to phylum level to include the two classes Ectoprocta and Entoprocta. However, in 1869 Hinrich Nitsche regarded the two groups as quite distinct for a variety of reasons, and coined the name "Ectoprocta" for Ehrenberg's "Bryozoa". Despite their apparently similar methods of feeding, they differed markedly anatomically; in addition to the different positions of the anus, ectoprocts have hollow tentacles and a coelom, while entoprocts have solid tentacles and no coelom. Hence the two groups are now widely regarded as separate phyla, and the name "Bryozoa" is now synonymous with "Ectoprocta". This has remained the majority view ever since, although most publications have preferred the name "Bryozoa" rather than "Ectoprocta". Nevertheless, some notable scientists have continued to regard the "Ectoprocta" and Entoprocta as close relatives and group them under "Bryozoa".

The ambiguity about the scope of the name "Bryozoa" led to proposals in the 1960s and 1970s that it should be avoided and the unambiguous term "Ectoprocta" should be used. However, the change would have made it harder to find older works in which the phylum was called "Bryozoa", and the desire to avoid ambiguity, if applied consistently to all classifications, would have necessitated renaming of several other phyla and many lower-level groups. In practice, zoological naming of split or merged groups of animals is complex and not completely consistent. Works since 2000 have used various names to resolve the ambiguity, including: "Bryozoa", "Ectoprocta",  "Bryozoa (Ectoprocta)", and "Ectoprocta (Bryozoa)". Some have used more than one approach in the same work.

The common name "moss animals" is the literal meaning of "Bryozoa", from Greek βρυόν ('moss') and ζῷα ('animals'), based on the mossy appearance of encrusting species.

Until 2008 there were "inadequately known and misunderstood type species belonging to the Cyclostome Bryozoan family Oncousoeciidae." Modern research and experiments have been done using low-vacuum scanning electron microscopy of uncoated type material to critically examine and perhaps revise the taxonomy of three genera belonging to this family, including Oncousoecia, Microeciella, and Eurystrotos. This method permits data to be obtained that would be difficult to recognize with an optical microscope. The valid type species of Oncousoecia was found to be Oncousoecia lobulata. This interpretation stabilizes Oncousoecia by establishing a type species that corresponds to the general usage of the genus. Fellow Oncousoeciid Eurystrotos is now believed to be not conspecific with O. lobulata, as previously suggested, but shows enough similarities to be considered a junior synonym of Oncousoecia. Microeciella suborbicularus has also been recently distinguished from O. lobulata and O. dilatans, using this modern method of low vacuum scanning, with which it has been inaccurately synonymized with in the past. A new genus has also been recently discovered called Junerossia in the family Stomachetosellidae, along with 10 relatively new species of bryozoa such as Alderina flaventa, Corbulella extenuata, Puellina septemcryptica, Junerossia copiosa, Calyptotheca kapaaensis, Bryopesanser serratus, Cribellopora souleorum, Metacleidochasma verrucosa, Disporella compta, and Favosipora adunca.

Classification and diversity
Counts of formally described species range between 4,000 and 4,500. The Gymnolaemata and especially Cheilostomata have the greatest numbers of species, possibly because of their wide range of specialist zooids. Under the Linnaean system of classification, which is still used as a convenient way to label groups of organisms, living members of the phylum Bryozoa are divided into:

Fossil record

Fossils of about 15,000 bryozoan species have been found. Bryozoans are among the three dominant groups of Paleozoic fossils. Bryozoans with calcitic skeletons were a major source of the carbonate minerals that make up limestones, and their fossils are incredibly common in marine sediments worldwide from the Ordovician onward. However, unlike corals and other colonial animals found in the fossil record, Bryozoan colonies did not reach large sizes. Fossil bryozoan colonies are typically found highly fragmented and scattered; the preservation of complete zoaria is uncommon in the fossil record, and relatively little study has been devoted to reassembling fragmented zoaria. The largest known fossil colonies are branching trepostome bryozoans from Ordovician rocks in the United States, reaching 66 centimeters in height.

The oldest species with a mineralized skeleton occurs in the Lower Ordovician. It is likely that the first bryozoans appeared much earlier and were entirely soft-bodied, and the Ordovician fossils record the appearance of mineralized skeletons in this phylum. By the Arenigian stage of the Early Ordovician period, about, all the modern orders of stenolaemates were present, and the ctenostome order of gymnolaemates had appeared by the Middle Ordovician, about. The Early Ordovician fossils may also represent forms that had already become significantly different from the original members of the phylum. Ctenostomes with phosphatized soft tissue are known from the Devonian. Other types of filter feeders appeared around the same time, which suggests that some change made the environment more favorable for this lifestyle. Fossils of cheilostomates, an order of gymnolaemates with mineralized skeletons, first appear in the Mid Jurassic, about, and these have been the most abundant and diverse bryozoans from the Cretaceous to the present. Evidence compiled from the last 100 million years show that cheilostomatids consistently grew over cyclostomatids in territorial struggles, which may help to explain how cheilostomatids replaced cyclostomatids as the dominant marine bryozoans. Marine fossils from the Paleozoic era, which ended, are mainly of erect forms, those from the Mesozoic are fairly equally divided by erect and encrusting forms, and more recent ones are predominantly encrusting. Fossils of the soft, freshwater phylactolaemates are very rare, appear in and after the Late Permian (which began about ) and consist entirely of their durable statoblasts. There are no known fossils of freshwater members of other classes.

Evolutionary family tree
Scientists are divided about whether the Bryozoa (Ectoprocta) are a monophyletic group (whether they include all and only a single ancestor species and all its descendants), about what are the phylum's closest relatives in the family tree of animals, and even about whether they should be regarded as members of the protostomes or deuterostomes, the two major groups that account for all moderately complex animals.

Molecular phylogeny, which attempts to work out the evolutionary family tree of organisms by comparing their biochemistry and especially their genes, has done much to clarify the relationships between the better-known invertebrate phyla. However, the shortage of genetic data about "minor phyla" such as bryozoans and entoprocts has left their relationships to other groups unclear.

Traditional view
The traditional view is that the Bryozoa are a monophyletic group, in which the class Phylactolaemata is most closely related to Stenolaemata and Ctenostomatida, the classes that appear earliest in the fossil record. However, in 2005 a molecular phylogeny study that focused on phylactolaemates concluded that these are more closely related to the phylum Phoronida, and especially to the only phoronid species that is colonial, than they are to the other ectoproct classes. That implies that the Entoprocta are not monophyletic, as the Phoronida are a sub-group of ectoprocts but the standard definition of Entoprocta excludes the Phoronida.

In 2009 another molecular phylogeny study, using a combination of genes from mitochondria and the cell nucleus, concluded that Bryozoa is a monophyletic phylum, in other words includes all the descendants of a common ancestor that is itself a bryozoan. The analysis also concluded that the classes Phylactolaemata, Stenolaemata and Gymnolaemata are also monophyletic, but could not determine whether Stenolaemata are more closely related to Phylactolaemata or Gymnolaemata. The Gymnolaemata are traditionally divided into the soft-bodied Ctenostomatida and mineralized Cheilostomata, but the 2009 analysis considered it more likely that neither of these orders is monophyletic and that mineralized skeletons probably evolved more than once within the early Gymnolaemata.

Bryozoans' relationships with other phyla are uncertain and controversial. Traditional phylogeny, based on anatomy and on the development of the adult forms from embryos, has produced no enduring consensus about the position of ectoprocts. Attempts to reconstruct the family tree of animals have largely ignored ectoprocts and other "minor phyla", which have received little scientific study because they are generally tiny, have relatively simple body plans, and have little impact on human economies – despite the fact that the "minor phyla" include most of the variety in the evolutionary history of animals.

In the opinion of Ruth Dewel, Judith Winston, and Frank McKinney, "Our standard interpretation of bryozoan morphology and embryology is a construct resulting from over 100 years of attempts to synthesize a single framework for all invertebrates," and takes little account of some peculiar features of ectoprocts.

In ectoprocts, all of the larva's internal organs are destroyed during the metamorphosis to the adult form and the adult's organs are built from the larva's epidermis and mesoderm, while in other bilaterians some organs including the gut are built from endoderm. In most bilaterian embryos the blastopore, a dent in the outer wall, deepens to become the larva's gut, but in ectoprocts the blastopore disappears and a new dent becomes the point from which the gut grows. The ectoproct coelom is formed by neither of the processes used by other bilaterians, enterocoely, in which pouches that form on the wall of the gut become separate cavities, nor schizocoely, in which the tissue between the gut and the body wall splits, forming paired cavities.

Entoprocts
When entoprocts were discovered in the 19th century, they and bryozoans (ectoprocts) were regarded as classes within the phylum Bryozoa, because both groups were sessile animals that filter-fed by means of a crown of tentacles that bore cilia.

From 1869 onwards increasing awareness of differences, including the position of the entoproct anus inside the feeding structure and the difference in the early pattern of division of cells in their embryos, caused scientists to regard the two groups as separate phyla, and "Bryozoa" became just an alternative name for ectoprocts, in which the anus is outside the feeding organ. A series of molecular phylogeny studies from 1996 to 2006 have also concluded that bryozoans (ectoprocts) and entoprocts are not sister groups.

However, two well-known zoologists, Claus Nielsen and Thomas Cavalier-Smith, maintain on anatomical and developmental grounds that bryozoans and entoprocts are member of the same phylum, Bryozoa. A molecular phylogeny study in 2007 also supported this old idea, while its conclusions about other phyla agreed with those of several other analyses.

Grouping into the Lophophorata
By 1891 bryozoans (ectoprocts) were grouped with phoronids in a super-phylum called "Tentaculata". In the 1970s comparisons between phoronid larvae and the cyphonautes larva of some gymnolaete bryozoans produced suggestions that the bryozoans, most of which are colonial, evolved from a semi-colonial species of phoronid. Brachiopods were also assigned to the "Tentaculata", which were renamed Lophophorata as they all use a lophophore for filter feeding.

The majority of scientists accept this, but Claus Nielsen thinks these similarities are superficial. The Lophophorata are usually defined as animals with a lophophore, a three-part coelom and a U-shaped gut. In Nielsen's opinion, phoronids' and brachiopods' lophophores are more like those of pterobranchs, which are members of the phylum Hemichordata. Bryozoan's tentacles bear cells with multiple cilia, while the corresponding cells of phoronids', brachiopods' and pterobranchs' lophophores have one cilium per cell; and bryozoan tentacles have no hemal canal ("blood vessel"), which those of the other three phyla have.

If the grouping of bryozoans with phoronids and brachiopods into Lophophorata is correct, the next issue is whether the Lophophorata are protostomes, along with most invertebrate phyla, or deuterostomes, along with chordates, hemichordates and echinoderms.

The traditional view was that lophophorates were a mix of protostome and deuterostome features. Research from the 1970s onwards suggested they were deuterostomes, because of some features that were thought characteristic of deuterostomes: a three-part coelom; radial rather than spiral cleavage in the development of the embryo; and formation of the coelom by enterocoely. However the coelom of ectoproct larvae shows no sign of division into three sections, and that of adult ectoprocts is different from that of other coelomate phyla as it is built anew from epidermis and mesoderm after metamorphosis has destroyed the larval coelom.

Lophophorate molecular phylogenetics
Molecular phylogeny analyses from 1995 onwards, using a variety of biochemical evidence and analytical techniques, placed the lophophorates as protostomes and closely related to annelids and molluscs in a super-phylum called Lophotrochozoa. "Total evidence" analyses, which used both morphological features and a relatively small set of genes, came to various conclusions, mostly favoring a close relationship between lophophorates and Lophotrochozoa. A study in 2008, using a larger set of genes, concluded that the lophophorates were closer to the Lophotrochozoa than to deuterostomes, but also that the lophophorates were not monophyletic. Instead, it concluded that brachiopods and phoronids formed a monophyletic group, but bryozoans (ectoprocts) were closest to entoprocts, supporting the original definition of "Bryozoa".

They are the only major phylum of exclusively clonal animals, composed of modular units known as zooids. Because they thrive in colonies, colonial growth allows them to develop unrestricted variations in form. Despite this, only a small number of basic growth forms have been found and have commonly reappeared throughout the history of the bryozoa.

Ectoproct molecular phylogenetics
The phylogenetic position of the ectoproct bryozoans remains uncertain, but it remains certain that they belong to the Protostomia and more specifically to the Lophotrochozoa. This implies that the ectoproct larva is a trochophore with the corona being a homologue of the prototroch; this is supported from the similarity between the coronate larvae and the Type 1 pericalymma larvae of some molluscs and sipunculans, where the prototroch zone is expanded to cover the hyposphere.

A study of the mitochondrial DNA sequence suggests that the Bryozoa may be related to the Chaetognatha.

Feeding and excretion
Most species are filter feeders that sieve small particles, mainly phytoplankton (microscopic floating plants), out of the water. The freshwater species Plumatella emarginata feeds on diatoms, green algae, cyanobacteria, non-photosynthetic bacteria, dinoflagellates, rotifers, protozoa, small nematodes, and microscopic crustaceans. While the currents that bryozoans generate to draw food towards the mouth are well understood, the exact method of capture is still debated. All species also flick larger particles towards the mouth with a tentacle, and a few capture zooplankton (planktonic animals) by using their tentacles as cages. In addition the tentacles, whose surface area is increased by microvilli (small hairs and pleats), absorb organic compounds dissolved in the water. Unwanted particles may be flicked away by tentacles or shut out by closing the mouth. A study in 2008 showed that both encrusting and erect colonies fed more quickly and grew faster in gentle than in strong currents.

In some species the first part of the stomach forms a muscular gizzard lined with chitinous teeth that crush armored prey such as diatoms. Wave-like peristaltic contractions move the food through the stomach for digestion. The final section of the stomach is lined with cilia (minute hairs) that compress undigested solids, which then pass through the intestine and out through the anus.

There are no nephridia ("little kidneys") or other excretory organs in bryozoa, and it is thought that ammonia diffuses out through the body wall and lophophore. More complex waste products are not excreted but accumulate in the polypide, which degenerates after a few weeks. Some of the old polypide is recycled, but much of it remains as a large mass of dying cells containing accumulated wastes, and this is compressed into a "brown body". When the degeneration is complete, the cystid (outer part of the animal) produces a new polypide, and the brown body remains in the coelom, or in the stomach of the new polypide and is expelled next time the animal defecates.

Respiration and circulation
There are no respiratory organs, heart or blood vessels. Instead, zooids absorb oxygen and eliminate carbon dioxide through diffusion. Bryozoa accomplish diffusion through the use of either a thin membrane (in the case of anascans and some polyzoa) or through pseudopores located on the outer dermis of the zooid. The different bryozoan groups use various methods to share nutrients and oxygen between zooids: some have quite large gaps in the body walls, allowing the coelomic fluid to circulate freely; in others, the funiculi (internal "little ropes") of adjacent zooids connect via small pores in the body wall.

Reproduction and life cycles

Zooids of all phylactolaemate species are simultaneous hermaphrodites. Although those of many marine species are protandric, in other words function first as males and then as females, their colonies contain a combination of zooids that are in their male and female stages. In all species the ovaries develop on the inside of the body wall, and the testes on the funiculus connecting the stomach to the body wall. Eggs and sperm are released into the coelom, and sperm exit into the water through pores in the tips of some of the tentacles, and then are captured by the feeding currents of zooids that are producing eggs. Some species' eggs are fertilized externally after being released through a pore between two tentacles, which in some cases is at the tip of a small projection called the "intertentacular organ" in the base of a pair of tentacles. Others' are fertilized internally, in the intertentacular organ or in the coelom.

All phylactolaemates and stenolaemates, and most gymnolaemates, exhibit placentation, and has therefore lecithotrophic (non-feeding) larvae. Except for Cyclostomata and the small gymnolaemate family Epistomiidae, which are viviparous, all are brooders. Phylactolaemata brood their embryos in an internal brood sac, but Gymnolaemata both external membranous sacs, skeletal chambers (ovicells) and internal brooding sacs exist. The developing embryo relies on egg's yolk, extraembryonic nutrition (matrotrophy) or both.

In ctenostomes the mother provides a brood chamber for the fertilized eggs, and her polypide disintegrates, providing nourishment to the embryo. Stenolaemates produce specialized zooids to serve as brood chambers, and their eggs divide within this to produce up to 100 identical embryos.

Planktotrophic (feeding) larvae are only found in class Gymnolaemata: In the cheilostomatan suborder Malacostegina they are found in the two families Membraniporidae and Electridae, and in the three ctenostome families Alcyonidiidae, Farrellidae, and Hislopiidae. In addition there are a few unconfirmed records, like the solitary form Aethozoid where larvae has never been observed, but which is assumed to have planktotrophic larvae.

The cleavage of bryozoan eggs is biradial, in other words the early stages are bilaterally symmetrical. It is unknown how the coelom forms, since the metamorphosis from larva to adult destroys all of the larva's internal tissues. In many animals the blastopore, an opening in the surface of the early embryo, tunnels through to form the gut. However, in bryozoans the blastopore closes, and a new opening develops to create the mouth.

Bryozoan larvae vary in form, but all have a band of cilia round the body which enables them to swim, a tuft of cilia at the top, and an adhesive sac that everts and anchors them when they settle on a surface. Some gymnolaemate species produce cyphonautes larvae which have little yolk but a well-developed mouth and gut, and live as plankton for a considerable time before settling. These larvae have triangular shells of chitin, with one corner at the top and the base open, forming a hood round the downward-facing mouth. In 2006 it was reported that the cilia of cyphonautes larvae use the same range of techniques as those of adults to capture food. Species that brood their embryos form larvae that are nourished by large yolks, have no gut and do not feed, and such larvae quickly settle on a surface. In all marine species the larvae produce cocoons in which they metamorphose completely after settling: the larva's epidermis becomes the lining of the coelom, and the internal tissues are converted to a food reserve that nourishes the developing zooid until it is ready to feed. The larvae of phylactolaemates produce multiple polypides, so that each new colony starts with several zooids. In all species the founder zooids then grow the new colonies by budding clones of themselves. In phylactolaemates, zooids die after producing several clones, so that living zooids are found only round the edges of a colony.

Phylactolaemates can also reproduce asexually by a method that enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Throughout summer and autumn they produce disc-shaped statoblasts, masses of cells that function as "survival pods" rather like the gemmules of sponges. Statoblasts form on the funiculus connected to the parent's gut, which nourishes them. As they grow, statoblasts develop protective bivalve-like shells made of chitin. When they mature, some statoblasts stick to the parent colony, some fall to the bottom ("sessoblasts"), some contain air spaces that enable them to float ("floatoblasts"), and some remain in the parent's cystid to re-build the colony if it dies. Statoblasts can remain dormant for considerable periods, and while dormant can survive harsh conditions such as freezing and desiccation. They can be transported across long distances by animals, floating vegetation, currents and winds, and even in the guts of larger animals. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. Plumatella emarginata produces both "sessoblasts", which enable the lineage to control a good territory even if hard times decimate the parent colonies, and "floatoblasts", which spread to new sites. New colonies of Plumatella repens produce mainly "sessoblasts" while mature ones switch to "floatoblasts". A study estimated that one group of colonies in a patch measuring 1 m2 produced 800,000 statoblasts.

Cupuladriid Bryozoa are capable of both sexual and asexual reproduction. The sexually reproducing colonies (aclonal) are the result of a larval cupuladriid growing into an adult stage whereas the asexual colonies(clonal) are a result of a fragment of a colony of cupuladriids growing into its own colony. The different forms of reproduction in cupuladriids are achieved through a variety of methods depending on the morphology and classification of the zooid.

Habitats and distribution
Most marine species live in tropical waters at depths less than 100 m. However, a few have been found in deep-sea trenches, especially around cold seeps, and others near the poles.

The great majority of bryozoans are sessile. Typically, sessile bryozoans live on hard substrates including rocks, sand or shells. Encrusting forms are much the commonest of these in shallow seas, but erect forms become more common as the depth increases. An example of incrustation on pebbles and cobbles is found in the diverse Pleistocene bryozoans found in northern Japan, where fossils have been found of single stones covered with more than 20 bryozoan species. Sediments with smaller particles, like sand or silt, are usually unsuitable habitat for bryozoans, but tiny colonies have been found encrusting grains of coarse sand. Some bryozoan species specialize in colonizing marine algae, seagrasses, and even mangrove roots; the genus Amphibiobeania lives on the leaves of mangrove trees and is called "amphibious" because it can survive regular exposure to air at low tide.

There are a variety of "free-living" bryozoans that live un-attached to a substrate. A few forms such as Cristatella can move. Lunulitiform cheilostomes are one group of free-living bryozoans with mobile colonies. They form small round colonies un-attached to any substrate; colonies of the genus Selenaria have been observed to "walk" around using setae. Another cheilostome family, the Cupuladriidae, convergently evolved similarly shaped colonies capable of movement. When observed in an aquarium, Selenaria maculata colonies were recorded to crawl at a speed of one meter per hour, climb over each other, move toward light, and right themselves when turned upside-down. Later study of this genus showed that neuroelectrical activity in the colonies increased in correlation with movement toward light sources. It is theorized that the capacity for movement arose as a side effect when colonies evolved longer setae for unburying themselves from sediment. Other free-living bryozoans are moved freely by waves, currents, or other phenomena. An Antarctic species, Alcyonidium pelagosphaera, consists of floating colonies. The pelagic species is between 5.0 and 23.0 mm in diameter, has the shape of a hollow sphere and consists of a single layer of autozooids. It is still not known if these colonies are pelagic their whole life or only represents a temporarily and previously undescribed juvenile stage. Colonies of the species Alcyonidium disciforme, which is disc-shaped and similarly free-living, inhabit muddy seabeds in the Arctic and can sequester sand grains they have engulfed, potentially using the sand as ballast to turn themselves right-side-up after they have been overturned. Some bryozoan species can form bryoliths, sphere-shaped free-living colonies that grow outward in all directions as they roll about on the seabed.

In 2014 it was reported that the bryozoan Fenestrulina rugula had become a dominant species in parts of Antarctica. Global warming has increased the rate of scouring by icebergs, and this species is particularly adept at recolonizing scoured areas.

The phylactolaemates live in all types of freshwater environment – lakes and ponds, rivers and streams, and estuaries – and are among the most abundant sessile freshwater animals. Some ctenostomes are exclusively freshwater while others prefer brackish water but can survive in freshwater. Scientists' knowledge of freshwater bryozoan populations in many parts of the world is incomplete, even in some parts of Europe. It was long thought that some freshwater species occurred worldwide, but since 2002 all of these have been split into more localized species.

Bryozoans grow in clonal colonies. A larval Bryozoan settles on a hard substance and produces a colony asexually through budding. These colonies can grow thousands of individual zooids in a relatively short period of time. Even though colonies of zooids grow through asexual reproduction, Bryozoans are hermaphrodites and new colonies can be formed through sexual reproduction and the generation of free swimming larvae. When colonies grow too large, however, they can split in two. This is the only case where asexual reproduction results in a new colony separate from its predecessor. Most colonies are stationary. Indeed, these colonies tend to be settled on immobile substances such as sediment and coarse substances. There are some colonies of freshwater species such as Cristatella mucedo that are able to move slowly on a creeping foot.

Interactions with non-human organisms
Marine species are common on coral reefs, but seldom a significant proportion of the total biomass. In temperate waters, the skeletons of dead colonies form a significant component of shell gravels, and live ones are abundant in these areas. The marine lace-like bryozoan Membranipora membranacea produces spines in response to predation by several species of sea slugs (nudibranchs). Other predators on marine bryozoans include fish, sea urchins, pycnogonids, crustaceans, mites and starfish. In general marine echinoderms and molluscs eat masses of zooids by gouging pieces of colonies, breaking their mineralized "houses", while most arthropod predators on bryozoans eat individual zooids.

In freshwater, bryozoans are among the most important filter feeders, along with sponges and mussels. Freshwater bryozoans are attacked by many predators, including snails, insects, and fish.

In Thailand the introduced species Pomacea canaliculata (golden apple snail), which is generally a destructive herbivore, has wiped out phylactolaemate populations wherever it has appeared. P. canaliculata also preys on a common freshwater gymnolaemate, but with less devastating effect. Indigenous snails do not feed on bryozoans.

Several species of the hydroid family Zancleidae have symbiotic relationships with bryozoans, some of which are beneficial to the hydroids while others are parasitic. Modifications appear in the shapes of some these hydroids, for example smaller tentacles or encrustation of the roots by bryozoans. The bryozoan Alcyonidium nodosum protects the whelk Burnupena papyracea against predation by the powerful and voracious rock lobster Jasus lalandii. While whelk shells encrusted by the bryozoans are stronger than those without this reinforcement, chemical defenses produced by the bryozoans are probably the more significant deterrent.

In the Banc d'Arguin offshore Mauritania the species Acanthodesia commensale, which is generally growing attached to gravel and hard-substrate, has formed a facultative symbiotic relationship with hermit crabs of the species ''Pseudopagurus cf. granulimanus'' resulting in egg-size structures known as bryoliths. Nucleating on an empty gastropod shell, the bryozoan colonies form multilamellar skeletal crusts that produce spherical encrustations and extend the living chamber of the hermit crab through helicospiral tubular growth.

Some phylactolaemate species are intermediate hosts for a group of myxozoa that have also been found to cause proliferative kidney disease, which is often fatal in salmonid fish, and has severely reduced wild fish populations in Europe and North America.

Membranipora membranacea, whose colonies feed and grow exceptionally fast in a wide range of current speeds, was first noticed in the Gulf of Maine in 1987 and quickly became the most abundant organism living on kelps. This invasion reduced the kelp population by breaking their fronds, so that its place as the dominant "vegetation" in some areas was taken by another invader, the large alga Codium fragile tomentosoides. These changes reduced the area of habitat available for local fish and invertebrates. M. membranacea has also invaded the northwest coast of the US. A few freshwater species have been also found thousands of kilometers from their native ranges. Some may have been transported naturally as statoblasts. Others more probably were spread by humans, for example on imported water plants or as stowaways on ships.

Interaction with humans
Fish farms and hatcheries have lost stock to proliferative kidney disease, which is caused by one or more myxozoans that use bryozoans as alternate hosts.

Some fishermen in the North Sea have had to find other work because of a form of eczema (a skin disease) known as "Dogger Bank itch", caused by contact with bryozoans that have stuck to nets and lobster pots.

Marine bryozoans are often responsible for biofouling on ships' hulls, on docks and marinas, and on offshore structures. They are among the first colonizers of new or recently cleaned structures. Freshwater species are occasional nuisances in water pipes, drinking water purification equipment, sewage treatment facilities, and the cooling pipes of power stations.

A group of chemicals called bryostatins can be extracted from the marine bryozoan Bugula neritina. In 2001 pharmaceutical company GPC Biotech licensed bryostatin 1 from Arizona State University for commercial development as a treatment for cancer. GPC Biotech canceled development in 2003, saying that bryostatin 1 showed little effectiveness and some toxic side effects. In January 2008 a clinical trial was submitted to the United States National Institutes of Health to measure the safety and effectiveness of Bryostatin 1 in the treatment of Alzheimer's disease. However, no participants had been recruited by the end of December 2008, when the study was scheduled for completion. More recent work shows it has positive effects on cognition in patients with Alzheimer's disease with few side effects. About 1000 kg of bryozoans must be processed to extract 1 g of bryostatin, As a result, synthetic equivalents have been developed that are simpler to produce and apparently at least as effective. <!-- Hide until mined out; then delete ******

Anatomy
Bryozoan skeletons grow in a variety of shapes and patterns: mound-shaped, lacy fans, branching twigs, and even corkscrew-shaped. Their skeletons have numerous tiny openings, each of which is the home of a minute animal called a zooid. They also have a coelomate body with a looped alimentary canal or gut, opening at the mouth and terminating at the anus. They feed with a specialized, ciliated structure called a lophophore, which is a crown of tentacles surrounding the mouth. Their diet consists of small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish. Bryozoans do not have any defined respiratory, or circulatory systems due to their small size. However, they do have a simple nervous system and a hydrostatic skeletal system. Several studies have been undertaken on the crystallography of bryozoan skeletons, revealing a complex fabric suite of oriented calcite or aragonite crystallites within an organic matrix – see for example Hall et al. (2002).

The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water which drives water together with entrained food particles (mainly phytoplankton) towards the mouth. The gut is U-shaped, and consists of a pharynx which passes into the esophagus, followed by the stomach, which has three parts: the cardia, the caecum, and the pylorus. The pylorus leads to an intestine and a short rectum terminating at the anus, which opens outside the lophophore. In some groups, notably some ctenostomes, a specialized gizzard may be formed from the proximal part of the cardia. Gut and lophophore are the principal components of the polypide. Cyclical degeneration and regeneration of the polypide is characteristic of marine bryozoans. After the final polypide degeneration, the skeletal aperture of the feeding zooid may become sealed by the secretion of a terminal diaphragm. In many bryozoans only the zooids within a few generations of the growing edge are in an actively feeding state; older, more proximal zooids (e.g. in the interiors of bushy colonies) are usually dormant.

Because of their small size, bryozoans have no need of a blood system. Gaseous exchange occurs across the entire surface of the body, but particularly through the tentacles of the lophophore.

Bryozoans can reproduce both sexually and asexually. All bryozoans, as far as is known, are hermaphroditic (meaning they are both male and female). Asexual reproduction occurs by budding off new zooids as the colony grows, and is the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. A colony formed this way is composed entirely of clones (genetically identical individuals) of the first animal, which is called the ancestrula.

One species of bryozoan, Bugula neritina, is of current interest as a source of cytotoxic chemicals, bryostatins, under clinical investigation as anti-cancer agents.

Fossils
Fossil bryozoans are found in rocks beginning in the Early Ordovician as part of the Ordovician radiation. They were often major components of Ordovician seabed communities and, like modern-day bryozoans, played an important role in sediment stabilization and binding, as well as providing sources of food for other benthic organisms. During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds. Bryozoan fossil record comprises more than 1,000 described species. It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some other reason; perhaps they evolved from a phoronid-like ancestor at about this time.

Bryozoans are important members of sclerobiont (organisms which dwell on hard substrates such as shells and rocks) communities in the fossil record and in the Recent. For a review of sclerobiont evolution, history and ecology, see Taylor & Wilson (2003).

Most fossil bryozoans have mineralized skeletons. The skeletons of individual zooids vary from tubular to box-shaped and contain a terminal aperture from which the lophophore is protruded to feed. No pores are present in the great majority of Ordovician bryozoans, but skeletal evidence shows that epithelia were continuous from one zooid to the next.

With regard to the bryozoan groups lacking mineralized skeletons, the statoblasts of freshwater phylactolaemates have been recorded as far back as the Permian, and the ctenostome fossils date from the Triassic.

One of the most important events during bryozoan evolution was the acquisition of a calcareous skeleton and the related change in the mechanism of tentacle protrusion. The rigidity of the outer body walls allowed a greater degree of zooid contiguity and the evolution of massive, multiserial colony forms.

Classification
The bryozoans were formerly considered to contain two subgroups: the ectoprocta and the entoprocta, based on the similar bodyplans and mode of life of these two groups. (Some researchers also included the Cycliophora, which are thought to be closely related to the entoprocta.) However, the ectoprocta are coelomate (possessing a body cavity) and their embryos undergo radial cleavage, while the entoprocta are acoelemate and undergo spiral cleavage. Molecular studies are ambiguous about the exact position of the entoprocta, but do not support a close relationship with the ectoprocta. For these reasons, the entoprocta are now considered a phylum of their own. The removal of the 150 species of entoprocta leaves bryozoa synonymous with ectoprocta; some authors have adopted the latter name for the group, but the majority continue to use the former.

The closest relations of the bryozoans appear to be the brachiopods. The sister group to this clade is still unclear but this seems most likely to be the phoronids.
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title: Bryozoa


text:
Biennial plant

A biennial plant is a flowering plant that, generally in a temperate climate, takes two years to complete its biological life cycle.

Background
In its first year, the biennial plant undergoes primary growth, during which its vegetative structures (leaves, stems, and roots) develop. Usually, the stem of the plant remains short and the leaves are low to the ground, forming a rosette. After one year's growing season, the plant enters a period of dormancy for the colder months. Many biennials require a cold treatment, or vernalization before they will flower. During the next spring or summer, the stem of the biennial plant elongates greatly, or "bolts". The plant then flowers, producing fruits and seeds before it finally dies. There are far fewer biennials than either perennial plants or annual plants.

Biennials do not always follow a strict two-year life cycle and the majority of plants in the wild can take 3 or more years to fully mature. Rosette leaf size has been found to predict when a plant may enter its second stage of flowering and seed production. Alternatively, under extreme climatic conditions, a biennial plant may complete its life cycle rapidly (e.g., in three months instead of two years). This is quite common in vegetable or flower seedlings that were vernalized before they were planted in the ground. This behavior leads to many normally biennial plants being treated as annuals in some areas. Conversely, an annual grown under extremely favorable conditions may have highly successful seed propagation, giving it the appearance of being biennial or perennial. Some short-lived perennials may appear to be biennial rather than perennial. True biennials flower only once, while many perennials will flower every year once mature.

Biennials grown for flowers, fruits, or seeds are grown for two years, whereas those grown for edible leaves or roots are harvested after one year—and are not kept a second year to run to seed.

Examples of biennial plants are members of the onion family including leek, some members of the cabbage family, common mullein, parsley, fennel, Lunaria, silverbeet, black-eyed Susan, sweet William, colic weed, carrot, and some hollyhocks. Plant breeders have produced annual cultivars of several biennials that will flower the first year from seed, for example, foxglove and stock.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Biennial plant


text:
Bay leaf

The bay leaf is an aromatic leaf commonly used as a herb in cooking. It can be used whole, either dried or fresh, in which case it is removed from the dish before consumption, or less commonly used in ground form. The flavor that a bay leaf imparts to a dish has not been universally agreed upon, but many agree it is a subtle addition.

Bay leaves come from various plants and are used for their distinctive flavor and fragrance. The most common source is the bay laurel (Laurus nobilis). Other types include California bay laurel, Indian bay leaf, West Indian bay laurel, and Mexican bay laurel. Bay leaves contain essential oils, such as eucalyptol, terpenes, and methyleugenol, which contribute to their taste and aroma.

Bay leaves are used in various cuisines around the world, including Indian, Filipino, European, and Caribbean. They are typically used in soups, stews, meat, seafood, and vegetable dishes. The leaves should be removed from the cooked food before eating as they can be abrasive in the digestive tract.

Bay leaves are used as an insect repellent in pantries and as an active ingredient in killing jars for entomology. In Eastern Orthodoxy liturgy, they are used to symbolize Jesus' destruction of Hades and freeing of the dead.

While some visually similar plants have poisonous leaves, bay leaves are not toxic and can be eaten without harm. However, they remain stiff even after cooking and may pose a choking hazard or cause harm to the digestive tract if swallowed whole or in large pieces. Canadian food and drug regulations set specific standards for bay leaves, including limits on ash content, moisture levels, and essential oil content.

Chemical constituents
The leaves of the European / Mediterranean plant Laurus nobilis contain about 1.3% essential oils (ol. lauri folii), consisting of 45% eucalyptol, 12% other terpenes, 8-12% terpinyl acetate, 3–4% sesquiterpenes, 3% methyleugenol, and other α- and β-pinenes, phellandrene, linalool, geraniol, terpineol, and also contain lauric acid.

Taste and aroma
If eaten whole, Laurus nobilis bay leaves are pungent and have a sharp, bitter taste. As with many spices and flavourings, the fragrance of the bay leaf is more noticeable than its taste. When the leaf is dried, the aroma is herbal, slightly floral, and somewhat similar to oregano and thyme. Myrcene, a component of many essential oils used in perfumery, can be extracted from this bay leaf. They also contain eugenol.

Uses
In Indian cuisine, bay laurel leaves are sometimes used in place of Indian bay leaf, although they have a different flavour. They are most often used in rice dishes like biryani and as an ingredient in garam masala. Bay leaves are called tezpattā (तेज़पत्ता, in Hindi), Tejpātā (তেজপাতা) in Bengali, তেজ পাত in Assamese and usually rendered into English as Tej Patta.

In the Philippines, dried bay laurel leaves are used in several Filipino dishes, such as menudo, beef pares, and adobo.

Bay leaves were used for flavouring by the ancient Greeks. They are a fixture in the cooking of many European cuisines (particularly those of the Mediterranean), as well as in the Americas. They are used in soups, stews, brines, meat, seafood, vegetable dishes, and sauces. The leaves also flavour many classic French and Italian dishes. The leaves are most often used whole (sometimes in a bouquet garni) and removed before serving (they can be abrasive in the digestive tract). Thai and Laotian cuisine employs bay leaf (ใบกระวาน, bai kra wān) in a few Arab-influenced dishes, notably massaman curry.

Bay leaves can also be crushed or ground before cooking. Crushed bay leaves impart more fragrance than whole leaves, but are more difficult to remove and thus they are often used in a muslin bag or tea infuser. Ground bay laurel may be substituted for whole leaves and does not need to be removed, but it is much stronger.

Bay leaves are also used in the making of jerk chicken in the Caribbean Islands. The bay leaves are soaked and placed on the cool side of the grill. Pimento sticks are placed on top of the leaves, and the chicken is placed on top and smoked. The leaves are also added whole to soups, stews, and other Caribbean dishes.

Bay leaves can also be used scattered in a pantry to repel meal moths, flies, and cockroaches. Mediouni-Ben Jemaa and Tersim 2011 find the essential oil to be usable as an insect repellent.

Bay leaves have been used in entomology as the active ingredient in killing jars. The crushed, fresh, young leaves are put into the jar under a layer of paper. The vapors they release kill insects slowly but effectively and keep the specimens relaxed and easy to mount. The leaves discourage the growth of molds. They are not effective for killing large beetles and similar specimens, but insects that have been killed in a cyanide killing jar can be transferred to a laurel jar to await mounting. There is confusion in the literature about whether Laurus nobilis is a source of cyanide to any practical extent, but there is no evidence that cyanide is relevant to its value in killing jars. It certainly is rich in various essential oil components that could incapacitate insects in high concentrations; such compounds include 1,8-cineole, alpha-terpinyl acetate, and methyl eugenol. It also is unclear to what extent the alleged effect of cyanide released by the crushed leaves has been mis-attributed to Laurus nobilis in confusion with the unrelated Prunus laurocerasus, the so-called cherry laurel, which certainly does contain dangerous concentrations of cyanogenic glycosides together with the enzymes to generate the hydrogen cyanide from the glycocides if the leaf is physically damaged.

Bay leaves are used in Eastern Orthodoxy liturgy. To mark Jesus' destruction of Hades and freeing of the dead, parishioners throw bay leaves and flowers into the air, letting them flutter to the ground.

Safety
Some members of the laurel family, as well as the unrelated but visually similar mountain laurel and cherry laurel, have leaves that are poisonous to humans and livestock. While these plants are not sold anywhere for culinary use, their visual similarity to bay leaves has led to the oft-repeated belief that bay leaves should be removed from food after cooking because they are poisonous. This is not true; bay leaves may be eaten without toxic effect. However, they remain unpleasantly stiff even after thorough cooking, and if swallowed whole or in large pieces they may pose a risk of harming the digestive tract or causing choking. Thus, most recipes that use bay leaves will recommend their removal after the cooking process has finished.

Canadian food and drug regulations
The Canadian government requires that the bay leaves contain no more than 4.5% total ash material, with a maximum of 0.5% of which is insoluble in hydrochloric acid. To be considered dried, they must contain 7% moisture or less. The oil content cannot be less than 1 milliliter per 100 grams of the spice.

title: Bay leaf


text:
Basis

Basis may refer to:

Finance and accounting

 * Adjusted basis, the net cost of an asset after adjusting for various tax-related items
 * Basis point, 0.01%, often used in the context of interest rates
 * Basis trading, a trading strategy consisting of the purchase of a security and the sale of a similar security
 * Basis of futures, the value differential between a future and the spot price
 * Basis (options), the value differential between a call option and a put option
 * Basis swap, an interest rate swap
 * Cost basis, in income tax law, the original cost of property adjusted for factors such as depreciation
 * Tax basis, cost of an asset

and technology

 * Basis function
 * Basis (linear algebra)
 * Dual basis
 * Orthonormal basis
 * Schauder basis
 * Basis (universal algebra)
 * Basis of a matroid
 * Generating set of an ideal:
 * Gröbner basis
 * Hilbert's basis theorem
 * Generating set of a group
 * Base (topology)
 * Change of basis
 * Greedoid
 * Normal basis
 * Polynomial basis
 * Radial basis function
 * Standard basis
 * Transcendence basis of a field extension
 * Basis database

Chemistry

 * Basis (crystal structure), the positions of the atoms inside the unit cell
 * Basis set (chemistry)
 * Dry basis, an expression of a calculation in which the presence of water is ignored

Organizations

 * Bangladesh Association of Software and Information Services
 * Basis Educational Group
 * Basis Schools, a group of schools in Arizona, Washington, D.C., and Texas
 * Basis Technology Corp., a text analytics company

People

 * Dimitris Basis, Greek singer
 * Liron Basis (born 1974), Israeli footballer

title: Basis


text:
Burgess Shale

The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fossil beds containing soft-part imprints.

The rock unit is a black shale and crops out at a number of localities near the town of Field in Yoho National Park and the Kicking Horse Pass. Another outcrop is in Kootenay National Park 42 km to the south.

History and significance

The Burgess Shale was discovered by palaeontologist Charles Walcott on 30 August 1909, towards the end of the season's fieldwork. He returned in 1910 with his sons, daughter, and wife, establishing a quarry on the flanks of Fossil Ridge. The significance of soft-bodied preservation, and the range of organisms he recognised as new to science, led him to return to the quarry almost every year until 1924. At that point, aged 74, he had amassed over 65,000 specimens. Describing the fossils was a vast task, pursued by Walcott until his death in 1927. Walcott, led by scientific opinion at the time, attempted to categorise all fossils into living taxa, and as a result, the fossils were regarded as little more than curiosities at the time. It was not until 1962 that a first-hand reinvestigation of the fossils was attempted, by Alberto Simonetta. This led scientists to recognise that Walcott had barely scratched the surface of information available in the Burgess Shale, and also made it clear that the organisms did not fit comfortably into modern groups.

Excavations were resumed at the Walcott Quarry by the Geological Survey of Canada under the persuasion of trilobite expert Harry Blackmore Whittington, and a new quarry, the Raymond, was established about 20 metres higher up Fossil Ridge. Whittington, with the help of research students Derek Briggs and Simon Conway Morris of the University of Cambridge, began a thorough reassessment of the Burgess Shale, and revealed that the fauna represented were much more diverse and unusual than Walcott had recognized. Many of the animals present had bizarre anatomical features and only the slightest resemblance to other known animals. Examples include Opabinia, with five eyes and a snout like a vacuum cleaner hose and Hallucigenia, which was originally reconstructed upside down, walking on bilaterally symmetrical spines.

With Parks Canada and UNESCO recognising the significance of the Burgess Shale, collecting fossils became politically more difficult from the mid-1970s. Collections continued to be made by the Royal Ontario Museum. The curator of invertebrate palaeontology, Desmond Collins, identified a number of additional outcrops, stratigraphically both higher and lower than the original Walcott quarry. These localities continue to yield new organisms faster than they can be studied.

Stephen Jay Gould's book Wonderful Life, published in 1989, brought the Burgess Shale fossils to the public's attention. Gould suggests that the extraordinary diversity of the fossils indicates that life forms at the time were much more disparate in body form than those that survive today, and that many of the unique lineages were evolutionary experiments that became extinct. Gould's interpretation of the diversity of Cambrian fauna relied heavily on Simon Conway Morris's reinterpretation of Charles Walcott's original publications. However, Conway Morris strongly disagreed with Gould's conclusions, arguing that almost all the Cambrian fauna could be classified into modern day phyla.

The Burgess Shale has attracted the interest of paleoclimatologists who want to study and predict long-term future changes in Earth's climate. According to Peter Ward and Donald Brownlee in the 2003 book The Life and Death of Planet Earth, climatologists study the fossil records in the Burgess Shale to understand the climate of the Cambrian explosion. It can be used to predict what Earth's climate would look like 500 million years in the future as a warming and expanding Sun, combined with declining CO2 and oxygen levels, eventually heat the Earth toward temperatures not seen since the Archean Eon 3 billion years ago (before the first plants and animals appeared). This in turn furthers understanding of how and when the last living things on Earth could potentially die out. See also Future of the Earth.

After the Burgess Shale site was registered as a World Heritage Site in 1980, it was included in the Canadian Rocky Mountain Parks WHS designation in 1984.

In 2012, the discovery was announced of another Burgess Shale outcrop in Kootenay National Park to the south. In just 15 days of field collecting in 2013, 50 animal species were unearthed at the new site.

IUGS geological heritage site
In respect of the site being 'characterized by exceptional soft-tissue preservation, [and containing] the most complete fossil record of Cambrian (Wuliuan) marine ecosystems', the International Union of Geological Sciences (IUGS) included the 'Burgess Shale Cambrian Paleontological Record' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an 'IUGS Geological Heritage Site' as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'

Geological setting

The fossil-bearing deposits of the Burgess Shale correlate to the Stephen Formation, a collection of slightly calcareous dark mudstones, about old. The beds were deposited at the base of a cliff about 160 m tall, below the depth agitated by waves during storms. This vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which probably formed shortly before the deposition of the Burgess Shale. The precise formation mechanism is not known for certain, but the most widely accepted hypothesis suggests that the edge of the Cathedral Formation reef became detached from the rest of the reef, slumping and being transported some distance – perhaps kilometers – away from the reef edge. Later reactivation of faults at the base of the formation led to its disintegration from about. This would have left a steep cliff, the bottom of which would be protected from tectonic decompression because the limestone of the Cathedral Formation is difficult to compress. This protection explains why fossils preserved further from the Cathedral Formation are impossible to work with – tectonic squeezing of the beds has produced a vertical cleavage that fractures the rocks, so they split perpendicular to the fossils. The Walcott quarry produced such spectacular fossils because it was so close to the Stephen Formation – indeed the quarry has now been excavated to the very edge of the Cambrian cliff.

It was originally thought that the Burgess Shale was deposited in anoxic conditions, but mounting research shows that oxygen was continually present in the sediment. The anoxic setting had been thought to not only protect the newly dead organisms from decay, but it also created chemical conditions allowing the preservation of the soft parts of the organisms. Further, it reduced the abundance of burrowing organisms – burrows and trackways are found in beds containing soft-bodied organisms, but they are rare and generally of limited vertical extent. Brine seeps are an alternative hypothesis; see Burgess Shale type preservation for a more thorough discussion.

Stratigraphy

The Burgess Shale Formation comprises 10 members, the most famous being the Walcott Quarry Shale Member comprising the greater phyllopod bed.

Taphonomy and diagenesis
There are many other comparable Cambrian lagerstätten; indeed such assemblages are far more common in the Cambrian than in any other period. This is mainly due to the limited extent of burrowing activity; as such bioturbation became more prevalent throughout the Cambrian, environments capable of preserving organisms' soft parts became much rarer. (The pre-Cambrian fossil record of animals is sparse and ambiguous, cf ediacaran biota.)

Biota
The biota of the Burgess Shale appears to be typical of middle Cambrian deposits. Although the hard-part bearing organisms make up as little as 14% of the community, these same organisms are found in similar proportions in other Cambrian localities. This means that there is no reason to assume that the organisms without hard parts are exceptional in any way; many appear in other lagerstätten of different age and locations.

The biota consists of a range of organisms. Free-swimming (nectonic) organisms are relatively rare, with the majority of organisms being bottom dwelling (benthic) — either moving about (vagrant) or permanently attached to the sea floor (sessile). About two-thirds of the Burgess Shale organisms lived by feeding on the organic content in the muddy sea floor, while almost a third filtered out fine particles from the water column. Under 10% of organisms were predators or scavengers, although since these organisms were larger, the biomass was split equally among each of the filter feeding, deposit feeding, predatory and scavenging organisms.

Many Burgess Shale organisms represent stem group members of the modern animal phyla, though crown group representatives of certain phyla are also present.

Working with the Burgess Shale
The fossils of the Burgess Shale are preserved as black carbon films on black shales, and so are difficult to photograph; however, various photographic techniques can improve the quality of the images that can be acquired. Other techniques include backscatter SEM, elemental mapping and camera lucida drawing.

Once images have been acquired, the effects of decay and taphonomy must be accounted for before a correct anatomical reconstruction can be made. A consideration of the combination of characters allows researchers to establish the taxonomic affinity.

title: Burgess Shale