1 STRUCTURE AND PHYSIOLOGY OF ANIMALS 33 



^on.il'°.I?*'°'' l*^*" '^Tf^'^ food-substances. Into this part are 

 ?l?r^ ff secretions of the various digestive glands, which act on 

 solnht Tv. ^^g/-«t'^^*^«f ^^^ f°'^'^ «° ^^ to render them more 

 ,,nS!nV fr^ *''^ ^™'"^ membrane of this part the digested 

 uutnent matter passes, to enter the blood-system. This region 

 nay present a number of subdivisions; neariy always there are 

 Zt^ine ^'"'' *^® stomach, and a narrow tube, the 



The egestive or effere7it region of the alimentary canal is the 

 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 



Fig. 24.— General viuw of the viscera of a male Trog, from the right side, a, stomach ; b, urinary 

 bladder ; c, small intestine ; cl, cloacal aperture ; d. large intestine ; e, liver ; t\ bile-duct ; 

 f, gall-bladder ; h, spleen ; i, lung ; k, larynx ; /, fat-body ; m, testis ; n, ureter';' o, kidney • 

 p, pancreas; a, cerebral hemisphere; sp, spinal cord; t, tongue; n, auricle; iir, urostyle - 

 r, ventricle ; vn, vesicula seminalis ; w, optic lobe ; x, cerebellum ; ?/, Eustachian recess ■ 

 2, nasal sac. (From ^Marshall.) ' 



mainly for the passage to the anal opening of the fceces 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 ejnthelium,. The 

 form and arrangement of the cells of this epithelium vary greatly 

 in different groups of animals. Usually, they are vertically 

 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 

 forth processes of their protoplasm (Fig. 11,A,), 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 



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