148 



COLLEGE ZOOLOGY 



cu. 



FIG. 96. Sense organ 



Just beneath the statocyst is a ciliated area supposed to be 



sensory in function, and on either side is a ciliated prolongation 



called the polar field (Fig. 93, B, Pf). 



Ctenophores are hermaphroditic. The ova are formed on one 



side and the spermatozoa on the other side of each meridional 



canal just beneath the ciliated bands 

 (Fig. 93, A, 5). The germ-cells pass into 

 the infundibulum and thence to the out- 

 -ot. side through the mouth. The fertilized 

 eggs develop directly into the adult 

 without the intervention of an asexual 

 generation as in many ccelenterates. 



The cellular layers of ctenophores con- 

 "titute a very small part of the body, 



cu, cupule formed of fused most of it being composed of the trans- 



(Fr^La^SCat P. arent ^^ &*' The thin 

 ise.) ciliated ectoderm covers the exterior and 



lines the stomodaeum; and the entoderm, 



also ciliated, lines the infundibulum and the canals to which 

 it gives rise. The muscle fibers which lie just beneath the 

 ectoderm and entoderm are derived from the mesoderm cells of 

 the embryo. Ctenophores are therefore triploblastic animals, 

 and represent a higher grade of development than that of the 

 ccelenterates. 



Definition. -- Phylum CTENOPHORA. SEA WALNUTS or 

 COMB JELLIES. -- Triploblastic animals; radial combined with 

 bilateral symmetry ; eight radially arranged rows of paddle 

 plates. 



The CTENOPHORA differ from the ccelenterates in several 

 important respects besides the presence of a distinct mesoderm. 

 With one probable exception, ctenophores do not possess 

 nematocysts, and the adhesive cells (Fig. 95) which take their 

 place are not homologous to nematocysts. Their ciliated bands, 

 aboral sense-organs, and pronounced biradial symmetry are 

 peculiarities which warrant placing ctenophores in a phylum 



