ALIMENTARY CANAL 



263 



folds, which are sometimes found in the oesophagus, stomach, and 

 rectum, often disappear on distension, and probably merely 

 provide for the enlargement of these cavities during the degluti- 

 tion of relatively large prey, or for the accumulation of faeces. 

 On the other hand, the permanent and often complicated folds of 

 the intestinal mucous membrane are probably related to an 

 increase in the secretive or absorptive area of this portion of the 

 alimentary canal. In the stomach the mucous membrane is 

 usually smooth, rarely, as in the " Electric Eel " (Gymnotus), 

 reticulate. In the intestine the folds assume a highly character- 

 istic and often complicated disposition.^ In the Cyclostomata 



€>§.% 



€0 



tm 







Fig. 158. — The intestinal mucous membrane of different Fishes, to show the transition 

 from simple longitudinal and transverse folds to crypts. A, Of an Elasmobranch ; 

 B, C, and D, of various Teleosts. (After Wiedersheim.) 



the folds are simple and longitudinally arranged. In Elasmo- 

 branchs (Fig. 158, A), obliquely transverse folds are present in 

 addition, and, uniting with the longitudinal ridges, bound linear 

 depressions. 



In various Teleostomi (Fig. 158, B, C, D), the union of the 

 two series of folds becomes more or less retiform, and the network 

 of intersecting ridges bounds a series of deep tubular crypts which 

 appear to penetrate to a considerable distance into the intestinal 

 waU, and possibly foreshadow the characteristic Lieberkiihn's 

 glands of Mammalia. Crypts may also be found in the stomach, 

 where they receive the apertures of the gastric glands, as in 

 Amiurus, but more usually they are restricted to the intestine. 

 In the Dipnoi (e.g. Protopterus) the mucous membrane of the 



^ "Wiedersheim, Lehrb. d. vergl. Anat. d. Wirhelthiere, ed. ii. Jena, 1886, p. 

 576. 



