TOLYZOA. 



561 



the lining of the alimentary canal, the ganglion, and the covering 

 of the tentacles and the tentacle-sheath (Fig. 451). 



The muscles, funiculus, and somatic mesoderm of the animal are 

 derived from other cells of the internal mass, while the body-cavity 

 comes from the space in the same mass. 



In the Pliyladolaemata the embryo develops in the brood pouch 

 into a vesicle, the outer wall of which is ectoderm, the inner 

 mesoderm, and the cavity coelom. The larva which issues, 

 either by the degeneration 

 of the polypide or the open- 

 ing out of the zooecium, is 

 covered with cilia by means 

 of which it swims. It is the 

 primary zooecium : during its 

 free-swimming life it develops 

 a complete polypide, or it may 

 be two polypides (Plumatella), 



or even a greater number 



FIG. 451. Diagram showing tlie developing first 

 polypide. in connection with the wall of the 

 primary zooecium. a anus ; rf rudiment of 

 stomach ; ee wall of primary zooecium (endo- 

 cyst) ; in mouth ; n rudiment of ganglion ; 

 r retractor muscle ; t tentacles ; ts tentacle- 

 sheath. (From Korschelt and Heider.) 



(Cristatella), before fixation 



takes place ; thus differing 



from the primary zooecium 



of the Gymnolaemata, which 



does not develop the polypide 



until after fixation. As in other 



forms, an invagination occurs, 



from the walls of which the polypide is developed, i.e. the lining of 



the alimentary canal,* the covering of the tentacles and tentacle-sheath, 



and the nerve ganglion are alike derived from so-called ectoderm. 



From the foregoing account a certain number of conclusions may 

 be drawn. (1) The free-swimming larva of the Ectoproda is a 

 trochosphere with a preoral ciliated ring, and an apical sense-organ 

 the aboral disc, f (2) The larva attaches itself by its oral face, and 

 becomes the primary zooecium. (3) The larval organs disappear in 

 the primary zooecium. (4) The formation of the adult organs 

 (except the body-wall) is either to be looked upon as a case of 



* In a recent work on the development of Plumatella (Leuckart and Chun's 

 Biblioth-cca Zoologica, heft 23, 1897) Braem states that endoderm tissue can 

 be made out in the embryo. 



t It is not quite certain that the aboral disc is the apical plate, but if 

 Banner's observation that it is connected with nerve tissue (Fig. 447) is correct, 

 and further, if Prouho is right in describing its degeneration in the primary 

 zooecium (Note, p. 560), this conclusion may reasonably be drawn. 



2 o 



