THE METAZOA 



69 



ment. 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. 65), partly by 

 certain large 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 blood, to be distributed 



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

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

 intestine; e, liver; f, bile duct; g, gall bladder; k, spleen; /, lung; k, larynx; 

 /, fit body; tn, testis; , ureter: o, kidney; /, pancreas; s, cerebral hemi- 

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

 v.s, vesicula seminalis; <w, optic lobe; x, cerebellum; y, Eustachian recess; 

 2, nasal sac. (From Marshall.) 



throughout the body. The insoluble and indigestible ingre- 

 dients of the food pass on through the posterior part of the 

 intestine, and reach the exterior through the anal aperture 

 as the faces. 



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, 



