TISSUE-DIFFERENTIATION 



105 



cally all animals there is a central tubular cavity into 

 which the food is taken to be dissolved or digested 

 by the secretions elaborated for this purpose. In the 



FIG. 37. Diagrams of various types of digestive systems : A, Hydra 

 in which there is no distinction between body cavity and digestive 

 cavity ; B, a longitudinal section and C, a horizontal section of a (poly- 

 clad) flatworm in which there is no body cavity ; and but one opening 

 to the many-branched digestive cavity ; D, anterior end of earthworm ; 

 E, section of snail ; F, alimentary canal of human being ; m, mouth ; 

 ph, pharynx ; en, enteron or digestive cavity ; st, stomach ; cr, crop ; 

 g, gizzard ; i, intestine ; s.i., small intestine ; c, colon ; r, rectum. 



simplest of the Metazoa (Coelenterata and "flat- 

 worms ") this sac has but one external opening 

 through which all the food is taken in and from which 

 all the non-utilizable parts are ejected (fig. 37). In 



