METAZOA VERMES. 287 



POLYZOA. 



The young stages of Polyzoa, also called Bryozoa, are 

 similar to those of Brachiopods, which is one good reason 

 for placing these two groups near each other. The Poly- 

 zoa develop from a trochophore and with one exception 

 become colonial forms. Their calcareous skeletons, con- 

 sisting of numberless little cavities or chambers, are often 

 seen encrusting seaweed. Other genera are like minia- 

 ture trees, branching coral, and the like. 



Loxosoma is the only single Polyzoan known, but 

 inasmuch as it is a commensal living with other animals, 

 the strong probability is that, though single, it is a spe- 

 cialized form, a reduced descendant of some colonial 

 ancestral species. For this reason we consider Pedicel- 

 lina (No. 710, P. cernua Pall.) as the more primitive. 

 Here we have a comparatively simple colony. No. 710, 

 a-h, represents eight members ; (a) and (b) are very 

 young stages ; (c) is also young with its tentacles drawn 

 in ; (d) is a vertical section showing mouth (m), stomach 

 (.r), and anus (an) ; (e) and (f) have the tentacles ex- 

 panded ; (g) is just losing its old cup-like body wall or 

 calyx and another bud is beginning to grow ; in (h) the 

 primary calyx has been lost and a new one is developing. 



It is seen that the alimentary canal is complete, and 

 that it makes a turn, bringing the anus near the mouth 

 within the circle of tentacles. This position of the anus 

 is characteristic of the generalized Polyzoa, since it is 

 found outside the circle of tentacles in the specialized 

 forms. 



When the colony of Pedicellina dies there are left 

 little stalks that rise from a creeping stolon, but in the 

 specialized Polyzoa an innumerable number of tiny cavi- 

 ties or zooecia remain, each one of which represents an 

 animal. 



Many Polyzoa are of exquisite beauty, as shown in 



