IK) 



PRINCIPLES OF ANIMAL BIOLOGY 



be taken as an example. In this animal (Fig. 60) the digestive organ 

 consists of a simple bag whose wall is composed of flagellated epithelial 

 cells. Branches of the bag extend out into the tentacles and into the 

 buds if the latter are present. A single opening, the mouth, leads into 

 this cavity. The coelenterates in general have a digestive organ of this 

 type and few modifications are shown. In none of them is there a second 

 opening, the anus, through which indigestible materials may be voided to 

 the exterior. The digestive apparatus of the coelenterates thus consists 

 of a single organ, and can hardly be called a system. 



Digestive Systems in the Invertebrates. — Among the flatworms 

 (Platyhelminthes) a true digestive system may be found. In the rhab- 



■ms 



Fig. 80. — Diagram of cross section of a vertebrate animal to show coelom. c, ccelom; 

 en, endoderm of intestine; j/c, germ colls; yc, germinal epithelium which hiter covers the 

 gonads and from which the germ colls issue; ?', cavity of intestine; 7n, myotome, or muscle 

 segment; ms, mesentery; nc, neural crest, from which nerves and ganglia develop; nd, 

 notochord, forerunner of the backbone; s, sjjinal cord. 



docoele turbellarians (Fig. 81) it consists of a mouth, a short pharynx, 

 and a sac-like intestine. In the triclad turbellarians the intestine (Fig. 

 82) is branched into three main parts of which one extends forward and 

 two backward to the extremities of the body. These main branches 

 have secondary branches which extend laterally toward the periphery. 

 In the roundworms (Nemathelminthcs) the digestive system consists of 

 a straight tube which extends through the length of the body from the 

 mouth to a posterior opening, the anus, through which undigested ma- 

 Un-ial is voided. In some species tooth-like structures may be developed 

 in the mouth cavity and nuiscular swellings may be present in the walls of 



