LIVER 



260 



In order to effect an increase of the absorptive surface, longi- 

 tudinal folds of the mucous membrane are formed, and a special 

 development of such a fold, taking a spiral course, may result in 

 the formation of a spiral valve (see p. 257). A further advance is 

 seen in the development of transverse folds between the longi- 

 tudinal ones these are already seen in Elasmobranchs and many 

 other Fishes ; and by still further modifications, crypts of varied 



ft ! ! 



c 



D 



E 



m 



m 



\V'x 



FIG. 215. SEMIDIAGRAMMATIC FIGURES OF THE Mucous MEMBRANE OF THE 

 INTESTINE OF FISHES, SHOWING INTERMEDIATE FORMS BETWEEN LONGITU- 

 DINAL FOLDS AND ROUND CRYPTS. 



A, Petromyzon, showing the distinct spiral fold ; B, an Elsmobranch C to E, 



various Teleosts. 



form and depth are produced, into which open the microscopic 

 glands, when present (comp. Fig. 215). 



Finger-shaped outgrowths or villi of the mucous membrane of 

 the intestine are first plainly distinguishable in Amphibians (espe- 

 cially Anura) and are especially well developed in Mammals. 

 In addition to these, folds of varied forms, on the surface of which 

 the villi may be situated, occur from the Amphibia onwards : as 

 examples may be mentioned the valvuli conniventes of Mammals 

 and Birds. 



LIVER. 



The liver, the form of which is always closely adapted to that 

 of the surrounding parts, underlies to a greater or less extent the 

 ventral side of the intestinal tract, and is present in all the Craniata. 

 It arises as an outgrowth from the endodermic epithelium of the 

 intestine close to the junction of the latter with the stomach. 



In Amphioxus a simple sac -like csecum is present in this 



