OTHER CCELENTERATA 281 



The Ctenophora, or sea-walnuts (Fig. 135), are so specialized a 

 type that their structure must be described at length to be under- 

 stood. It will be recalled that they are sometimes classified as a 

 separate phylum of the Animal Kingdom instead of a class within 

 the Phylum Codenterata (cf. p. 237). In typical cases they swim 

 by means of eight rows of swimming plates formed by modifica- 

 tions of cilia. They are far less common than the jellyfishes, but 

 abound in some regions, including the JMediterranean Sea. 

 Mnemioysis leidiji and Pleurobrachia pileus of the Atlantic Coast 

 are the forms most familiar to American students. 



The Coelenterate Plan of Body. — The body plan that occurs in 

 coelenterates is important for comparison with that existing in 

 other phyla. Like hydra, the schematic coelenterate is sac-like, 

 with the walls composed of two cell layers, the ectoderm and the 

 endoderm, and with a mouth, which combines the functions 

 of mouth and anus in higher animals. The central cavity, 

 or cntcron, is comparable with the digestive tract in animals like 

 the frog and the earthworm. If comparisons can be made 

 between the cell layers, the ectoderm is comparable with the 

 epidennal layer of the skin, and the endoderm with the mucous 

 membrane of the digestive tract. Thus, the greater bulk of the 

 l:)ody, derived from what is called mesoderm in an animal like 

 a vertebrate, is not represented in the Coelenterata, which are 

 dipJobladic, or two-layered, animals, in contrast with animals that 

 are called triplohlastic or three-layered, because they have ecto- 

 derm, mesoderm, and endoderm. The gastrida stage, which occurs 

 in the development of many triplohlastic animals, has been 

 compared with the ccelenterate plan of body (cf. p. 242). 



