148 



COLLEGE ZOOLOGY 



Fig. g6. — Sense organ 

 of Hormiphora plumosa, a 

 ctenophore. Side view. 

 cu, cupule formed of fused 

 cilia ; ol, statolith mass. 

 (From Lankester's Treat- 

 ise.) 



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- 

 side through the mouth. The fertilized 

 eggs develop directly into the adult 

 without the intervention of an asexual 

 generation as in many coelenterates. 



The cellular layers of ctenophores con- 

 stitute a very small part of the body, 

 most of it being composed of the trans- 

 parent jelly-like mesoglea. The thin 

 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 

 coelenterates. 



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 coelenterates 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 



