| Phylactolaemata | |
|---|---|
 | |
| Zooid of a Phylactoaemata colony | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Bryozoa |
| Class: | Phylactolaemata |
| Order: | Plumatellida |
| Families | |
Phylactolaemata[1] is aclass of thephylumBryozoa whose members live only infreshwater environments. Like all bryozoans, theyfilter feed by means of an extensible "crown" ofciliated tentacles called alophophore, and like nearly all bryozoans (the only known exception beingMonobryozoon), they live in colonies, each of which consists ofclones of the founding member. Unlike those of some marine bryozoans, phylactolaemate colonies consist of only one type ofzooid, the feeding forms known as autozooids.[2] These are supported by anunmineralized "exoskeleton" made ofgelatinous material orprotein, secreted by the zooids.[3] The class contains only oneextantorder,Plumatellida.[4]
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.[5]
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Phylactolaemate colonies reproducesexually, and each member is a simultaneoushermaphrodite that functions as both male and female.[2] 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.[3] Among the classes of the bryozoans, the phylactoaemata have the most aberrant spermatozoon.[6] Throughout summer and autumn they produce disc-shapedstatoblasts, masses of cells that function as "survival pods" rather like thegemmules of sponges.[2] Statoblasts form on the funiculus (cord) connected to the parent's gut, which nourishes them.[3] As they grow, statoblasts develop protectivebivalve-like shells made ofchitin. When they mature, some types stick to the parent colony, some fall to the bottom, some contain air spaces that enable them to float,[2] and some remain in the parent's cystid (outer casing) to re-build the colony if it dies.[3] 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[2] and winds.[3] 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 1 square meter (11 sq ft) produced 800,000 statoblasts.[2]
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