The mucous membrane, including the submucosa, is frequently developed into variously arranged ingrowths projecting into the lumen of the alimentary canal; these are generally of the nature of longitudinal or transverse ridges, or a combination of the two, giving rise to retiform structures. The simple longitudinal {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 deglutition 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 characteristic and often complicated disposition.[241] In the Cyclostomata the folds are simple and longitudinally arranged. In Elasmobranchs (Fig. 158, A), obliquely transverse folds are present in addition, and, uniting with the longitudinal ridges, bound linear depressions. [Illustration: 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.)] 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 Lieberkühn'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 {264}stomach, and—excluding the Bursa Entiana where a number of oblique folds are present—of the intestine also, is, on the contrary, perfectly smooth.