POLYZOA 535 



to feed. The Cyphonautes is, then, the typical form (Fig. 395). It 

 •possesses a bivalve shell, each valve being triangular. The apical organ 

 and ciliated ring (corresponding to the prototroch) can be seen pro- 

 jecting from between the valves, and in addition there are various 

 characteristic organs, such as the internal sac, by which attachment is 

 effected, prior to metamorphosis, and the pyriform organ of unknown 

 function. On attachment the alimentary canal degenerates and the 

 first individual of the colony is formed by invagination froma polypide 

 bud consisting of an internal layer of ectoderm and an external of 



Fig. 395. Cyphonautes larva seen A, in side view, B, in oral view. al.c. ali- 

 mentary canal ; An. anus; ap.o. apical organ; ctl.r. ciliated ring; coe. coelom; 

 i.s. internal sac; M. mouth; p.o. pyriform organ; ve. vestibule. 



mesoderm. The ectoderm gives rise to the tentacles and tentacle 

 sheath, the ganglion and the alimentary canal of the new polypide. 

 A polypide bud which develops in exactly the same way is formed in 

 the course of regeneration after the formation of a brown body. 



In the Cyclostomata it is probable that the fertilized egg never de- 

 velops into a single individual but always into a large number by what 

 is known as embryonic fission, such as occurs in the parasitic Hymen- 

 optera. A much modified zooecium, the so-called ovicell, serves as a 

 brood pouch and in that the primary embryo is formed and attached 



