2l6 



CHORDATE ANATOMY 



ilates it, and extrudes what remains. The Porifera, though they have a 

 cloacal cavity, do their feeding essentially like Protozoa, each cell for itself. 

 The first real step in evolving a proper digestive system is taken by 

 the coelenterates. These, as their name affirms, have a cavity or enteron 

 which is the digestive tract. This has but one opening to the exterior, 

 which serves both as mouth and anus. See Fig. 375. 



Fig. 202. — Diagram of a vertebrate, a, anus; h, brain; df, dorsal fin; h, heart; i, 

 intestine; /, liver; m, mouth; n, nephridia; p, pancreas; pc, pericardium; pf, pectoral 

 fin; 5, stomach; sc, spinal cord; sp, spleen; vf, ventral fin. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



Most flatworms, like coelenterates, have a single opening to the 

 digestive cavity (enteron), and this opening serves as both mouth and 

 anus. A few species of flatworms, however, possess an anus — some indeed 

 have two ani — the invention of which therefore should be credited to 

 flatworms. Threadworms, with few exceptions, have both mouth and 



Fig. 203. — Spiral valve of Raia. Cartilaginous fishes increase the absorbing surface 

 of their intestine not by elongation, as is done by higher animals, but by a spiral fold in 

 theintestine. (From Kingsley's " Comparative Anatomy of Vertebrates," after Mayer.) 



anus, and their alimentary canal is separated from the muscular body- 

 wall by a space, a false body-cavity or pseudocoelom. The digestive 

 tube in threadworms is purely epithelial and non-muscular. 



A muscular digestive tube, one of the important steps in animal evolu- 

 tion, is contributed by the annelids. In these for the first time in the 

 phylogenesis of animals an epithelium-lined coelom or ''body-cavity" 



