Phylactolaemata is a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

of the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

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 long, they have a special feeding structure called a l ...

whose members live only in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

environments. Like all bryozoans, they

filter feed Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

by means of an extensible "crown" of

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

ted tentacles called a

lophophore The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.Monobryozoon ''Monobryozoon'' is a genus of bryozoans belonging to the family Monobryozoidae. The species of this genus are found in Northern Europe. Species: *''Monobryozoon ambulans'' *''Monobryozoon bulbosum'' *''Monobryozoon sandersi ''Monobryozoon ...

), they live in colonies, each of which consists of clones of the founding member. Unlike those of some marine bryozoans, phylactolaemate colonies consist of only one type of

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooi ...

, the feeding forms known as autozooids. These are supported by an unmineralized "

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

" made of

gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...

ous material or

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

, secreted by the zooids. The class contains only one

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

order, Plumatellida.

Fossil record

Phylactolaemata is regarded to have been the earliest group of bryozoans to evolve. However, because they did not have calcified skeletons, these early bryozoans would have had very low potential to fossilize. Fossils of phylactolaemate statoblasts, which consist of protective chitinous shells that serve as protection for dormant masses of cells holding the potential to grow new colonies, have been found rarely, dating back to the Permian. It is possible that the absence of statoblasts in earlier rocks is because statoblasts evolved as an adaptation for surviving in freshwater, and earlier phylactolaemates were marine.

Biology

Statoblastes de Cristatella mucedo (agglomérat) 05

Phylactolaemate colonies reproduce sexually, and each member is a simultaneous

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

that functions as both male and female. They also reproduce asexually by a method that is unique among bryozoans and enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Among the classes of the bryozoans, the phylactoaemata have the most aberrant spermatozoon. Throughout summer and autumn they produce disc-shaped

statoblast Statoblasts are a means to reproduce asexually by a method that is unique among bryozoans and enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Statoblasts are masses of cells that function as " ...

s, masses of cells that function as "survival pods" rather like the gemmules of sponges. Statoblasts form on the funiculus (cord) connected to the parent's gut, which nourishes them. As they grow, statoblasts develop protective bivalve-like shells made of

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

. When they mature, some types stick to the parent colony, some fall to the bottom, some contain air spaces that enable them to float, and some remain in the parent's cystid (outer casing) 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. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. A study estimated that one group of colonies in a patch produced 800,000 statoblasts.

References

{{Taxonbar, from1=Q4173968, from2=Q38957 Monotypic eukaryote classes Protostome classes