240 



COMPARATIVE ANATOMY. 



wall of the gut (B to E}. By a further development these crypts 

 become more tube-like and elongated, and give rise to the above- 

 mentioned gastric and intestinal glands (peptic glands and glands 

 of Lieberkiihn). 



B 



E 



FIG. 195. SEMIDIAGRAMMATIC FIGURES OF THE Mucous MEMBRANE OF THE 

 INTESTINE OF FISHES, SHOWING INTERMEDIATE FORMS BETWEEN LONGITUDINAL 

 FOLDS AND ROUND CRYPTS. 



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



various Teleosteans. 



The finger-like outgrowths of the mucosa (villi intestinales) 

 (Fig. 194, A, Zo, Zo) are to be looked at from the same physio- 

 logical point of view, that is, they have to do with increasing the 

 absorptive surface. They may be derived through numerous 

 intermediate forms from ordinary folds, and appear as distinct 

 papillae from the Sauropsida onwards, reaching their greatest 

 development in Mammals. 



APPENDAGES OF THE ALIMENTARY CANAL. 

 Liver. 



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

 of the body, underlies to a greater or less extent the ventral 

 side of the intestinal tract, and is present in every Vertebrate 

 (Amphioxus ?). It is united to the body-wall by a fold of the 

 peritoneum, and varies considerably in the number of its lobes. 

 We may nevertheless fix upon a ground-form consisting 

 of two lobes (Myxinoids) as the precedessor of the organ in all 

 Vertebrates. It always arises from the commencement of the small 

 intestine, and develops into a large vascular and glandular organ, 

 (Figs. 196 to 198) which gives rise to bile, and remains in com- 



