31 



ZOOLOGY 



SECT. 



posterior part of the intestine, in which digestion and absorption 

 do not go on, or only go on to a limited extent, and which serves 

 mainly for the passage to the anal opening of the faces or 

 unabsorbed effete matters of the food. 



The whole of the interior of the alimentary canal is lined 

 by a layer of cells the alimentary or enteric epithelium. The 

 form and arrangement of the cells of this epithelium vary greatly 

 different groups of animals. Usually, they are vertically 



m 



elongated, prismatic or columnar, or pyramidal in shape ; 

 frequently they are ciliated. In some lower forms, the cells lining 

 the alimentary cavity have the power, like Amoeba, of thrusting 



Fi<; 24. General view of the viscera of a male Frog, from the right Mile, a, stomach; l>, 

 urinary bladder; c, small intestine; cl, cloacal aperture; d, large intestine; e, liver; 

 /, bile-duct ; g, gall-bladder ; h, spleen ; ?, lung ; k, larynx ; /, fat-body ; m, testis ; 

 n, ureter ; o, kidney ; j>, pancreas ; s, cerebral hemisphere ; sp, spinal cord ; c, tongue ; 

 u, auricle ; ur, urostyle ; v, ventricle ; vs, vesiculaseminalis ; it; optic lobe ; x, cerebellum ; 

 y, Eustachian recess ; z, nasal sac. (From Marshall.) 



forth processes of their protoplasm (Fig. 11, h), and of taking minute 

 particles of food into their interior to become digested and absorbed 

 (intracellular digestion). Sometimes they are all more or less 

 active in secreting a fluid destined to act on the food and render 

 it more soluble ; sometimes this function is confined to certain of 

 the cells, which have a special form ; very often the secreting cells 

 line special little pouch-like, simple or branched glands, opening 

 by a passage or duct into the main cavity of the alimentary 

 canal. Besides these glands formed from specially modified cells 

 of the enteric epithelium there are nearly always present certain 

 large special glands, separate from the alimentary canal itself, but 

 opening into it by means of ducts. Of these the most generally- 

 occurring are the glands termed salivary glands, liver, and pancreas. 

 The salivary glands have the function of secreting a fluid called 

 the saliva, which, in many cases at least, has a special action on 

 starchy matters, converting them into sugar. The ducts of these 



