POLYZOA 



6ll 



then, the typical form (Fig. 420). It possesses a bivalve shell, each 

 valve being triangular. The apical organ and ciliated ring (correspond- 

 ing to the prototroch) can be seen projecting 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 from a polypide bud consisting of an internal layer of 





Fig. 420. Cyphonautes larva seen A, in side view, B, in oral view. al.c. ali- 

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

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



ectoderm and an external of 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 forma- 

 tion of a brown body. In the Endoprocta the larva fixes by its oral 

 surface and undergoes metamorphosis into the adult in the course 

 of which the mouth rotates upwards (compare Cirripedes, Fig. 256). 

 The alimentary canal does not degenerate. 



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



