

APPENDIX B 637 



Body wall of adults primarily diploblastic. However, the jellylike 

 mesoglea, separating the ectoderm from the entoderm, contains some 

 muscle cells that originate embryologically in such a manner as to suggest 

 that they are mesodermal. Hence the Ctenophora is often placed as the 

 most primitive of the triploblastic phyla. Radial combined with bilateral 

 symmetry (sometimes called biradial symmetry). A highly specialized 

 coelenteron consisting of one cavity that connects with a system of tubes 

 and canals, and another cavity (reached through the single opening, the 

 mouth), each cavity flattened on a plane at right angles to the other. 



Among the features peculiar to the phylum are the tentacles equipped with 

 adhesive cells instead of nematocysts and the possession of eight meridionally 

 arranged rows (combs) of swimming plates. 



The Ctenophora is a minor phylum composed of exclusively marine jellyfishlike 

 animals, formerly classified as Coelenterata. Ctenophora differ from coelenterates 

 in their general body form (bell-shaped or ribbonlike), their tendency toward the 

 triploblastic type of organization, the absence of nematocysts, and biradial 

 symmetry. There is no mistaking ctenophores seen in life: they are remarkably 

 transparent and are among the most beautiful of all sea animals. During the day, 

 the plate rows are irridescent with reflected light, and at night, the entire animal 

 may be vividly phosphorescent. 



One hundred species are known. 



Phylum V. PLATYHELMINTHES (plat' e hel min' thez; Greek, 

 platys, "broad," and helminthos, "worm"). 



The flatworms. Intermediate Metazoa, exhibiting the following 

 characters. 



Body triploblastic, much flattened dorsoventrally. Flatworms possess 

 a definite third layer of cells, the mesoderm, and have developed from 

 this layer various systems of organs. Bilateral symmetry. No true metam- 

 erism, unless the tapeworm be considered an "individual" rather than a 

 colony of individuals. 



Digestive system essentially a coelenteron, with a single opening, 

 the mouth. In many species, however, the coelenteron has been modified 

 into a highly branched gastrovascular system. No coelom; no anus. 

 Digestive system entirely lacking in the tapeworms, which absorb 

 through the body wall food already digested in the intestine of their host. 



Among the special features of the phylum are the presence of "flame cells" 

 in the excretory system and the fact that individuals are for the most part her- 

 maphroditic, having both male and female reproductive systems. 



The Platyhelminthes include both free-living and parasitic species. The former, 

 of which Planaria (class Turbellaria, Fig. 13.6) is an example, are the less numer- 

 ous and occur principally in fresh or salt water. The parasites include the flukes 

 (class Trematoda) and the tapeworms (class Cestoidea, or Cestoda). Most of 



