IX 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 
Fic. 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 
wall, 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 
1 Wiedersheim, Lehrb. d. vergl. Anat. d. Wirbelthiere, ed. ii. Jena, 1886, p. 
576. 
