78 ANIMAL BIOLOGY 



glimpse of the basic group of Coelenterates and what it has accom- 

 plished with the simple radially symmetrical body plan, with two 

 primary tissue layers, and an enteric cavity. The polyp type is 

 successful in its way, but its chief contribution was apparently in 

 affording the stem from which higher forms started along their 

 main path of ascent. (Fig. 297.) 



C. Flat worms 



Passing over a small phylum of marine Coelenterate-like ani- 

 mals, the Ctenophora, popularly called Sea-walnuts and Comb- 

 jellies, that have made a somewhat abortive attempt to establish 

 a body on the three primary layer plan, we come directly to the 

 first great group of triplorlastic animals, the Flatworms, con- 

 stituting the phylum Platyhelminthes. (Fig. 43.) 



On the lower surface of submerged stones near the edges of ponds 

 are usually many tiny black, gray, or white Flatworms, creeping 

 by cilia, known as Planaria. They are obviously radically different 

 in structure from a polyp since they exhibit rilateral instead of 

 radial symmetry: they have a broad anterior end with simple 

 eyes and brain, and a narrower posterior end, and so have right 

 and left sides. Strange to say, however, the mouth is situated not 

 in the 'head,' but behind the middle of the ventral surface of the 

 body, and functions both for the intake of food and the exit of 

 waste. The mouth, instead of opening into a sac-like enteron, 

 leads into a long, protrusible pharynx and this, in turn, into an 

 extremely branched intestine which extends throughout the body. 

 Somewhat similarly extend the excretory system, the male and 

 female reproductive systems, as well as the web-like nervous sys- 

 tem; each and all embedded in a continuous spongy mass of tissue 

 which is derived from a third primary germ layer, the mesoderm. 

 Thus the organ systems do not actually lie in a definite body 

 cavity. Since nearly every organ system extends throughout the 

 body, no circulatory system is required, and apparently oxygen 

 sufficient for the animal's need can diffuse through the tissues. 

 (Figs. 156, 161.) 



The genus Planaria is a member of the first class of the phylum, 

 known as the Turrellaria, the other classes being the Tre- 

 matoda, or Flukes, and the Cestoda, or Tapeworms. Both of 

 the latter have departed superficially somewhat widely in struc- 

 ture from the free-living Planarians and also have developed ex- 



