346 COMPARATIVE ANATOMY 



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. Complications then arise by the 

 development of transverse folds between the longitudinal ones 

 (these are already seen in Elasmobranchs and many other Fishes) ; 

 and by still further modifications, crypts of varied form and depth 

 are produced, into which open the microscopic glands, when 

 present (cf. Fig. 253). Finger-shaped outgrowths or villi of the 

 mucous membrane of the intestine are first plainly distinguish- 

 able in Amphibians (especially Anura) arid are especially well 

 developed in Mammals. 1 



Appendicular Organs of the Alimentary Canal. 



LIVER. 



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

 of the surrounding parts (Figs. 254 and 255), and which is typically 

 lobed, underlies to a greater or less extent the ventral side of the 

 intestinal tract, and is present in all the Craniata. It arises 



J)u 



FIG. 254. LIVER OF Rana esculenta. From the ventral side. 



Du, duodenum ; H, heart ; L, L l , L 2 , the different lobes of the liver ; M, 

 stomach. A gall-bladder is present, but is not indicated. 



in the form of a tubular gland, 2 as an outgrowth from the 

 endodermic epithelium of the intestine close to the junction of the 



1 The transverse folds of the mucous membrane occurring e.g. in Ratitse 

 and Mammalia in the small and large intestine are known respectively as plicae, 

 circulares (valvuloe, conniventes) a,ndplicce semilunares. 



2 Further differentiations occur, which gradually lead to a dendritic or net- 

 like arrangement of the gland-ductules, and this shows great variation in different 

 groups and in different stages of development. 



