THE METAZOA 



69 



nourishment. These digestive secretions are partly 

 produced by the cells of the epithelium of the canal, which 

 are modified to form unicellular or multicellular glands 

 (p. 64), partly by certain other special digestive glands, 

 salivary glands, liver and pancreas. The nutrient parts of 

 the food are by this means so acted upon that they are 

 ready to be absorbed, and in most animals pass into the 



FIG. 31. General view of the viscera of a male Frog, from the right side, rt, 

 stomach ; b, urinary bladder ; c, small intestine ; ct, cloacal aperture ; d, large 

 intestine ; e, liver ; f. bile duct ; , gall bladder ; h, spleen ; j, lung ; k, larynx ; 

 /, fat body ; m, testis ; . ureter ; o, kidney ; /, pancreas ; s, cerebral hemi- 

 sphere ; sp, spinal cord ; t, tongue ; K, auricle ; -ur, urostyle ; v, ventricle ; 

 v.s, vesicula seminalis ; m, optic lobe ; x, cerebellum ; y, Eustachu 

 z, nasal sac. (From Milnes Marshall.) 



Eustachian recess ; 



blood, to be distributed throughout the body. The in- 

 soluble and indigestible ingredients of the food pass on 

 through the posterior part of the intestine, and reach the 

 exterior through the anal aperture as theyfedw. 



A supply of oxygen is necessary for the carrying on of the 

 chemical changes in the tissues on which vital activity is 

 dependent. At the same time, as a result of these changes, 



