THE DIGESTIVE SYSTEM 



293 



ilates it, and extrudes what remains. Tlie 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 also the digestive tract. This has but one opening to the exterior, 

 which serves both as mouth and anus. 



Fig. 244. — Diagram of a vertebrate, a, anus; b, brain; c, coelom; da, dorsal aorta; 

 df, dorsal fin; g, gonad; gd, genital duct; h, heart; i, intestine; I, liver; m, mouth; n, 

 nephridia; o, oviduct; p, pancreas; pc, pericardium; pf, pectoral fin; ph, pharynx, with 

 gill clefts; s, stomach; sc, spinal cord; sp, spleen; u, ureter; vf, ventral fin. (From 

 Kingsley's "Comparative 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 



Fig. 245. — 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 

 the intestine. (From Kinglsey's " Comparative Anatomy of Vertebrates," after Mayer.) 



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

 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" 



