The intestinal canal in the osseous fishes is a simple tube (fig. 317, i), folded in sundry gyrations proportioned to its length; but in the cartilaginous families, such as the Sharks, the Kays, and the Sturgeons, it presents internally a very remarkable arrangement, evidently intended to increase the extent of surface over which the digested aliment may be spread, for the purpose of absorbing its nutritive portions. In these tribes a spiral valve (fig. 327, h) winds in close turns from the pyloric to the anal extremity of the capacious intestine; so that, although externally the intestine appears short in proportion to the size of the animal, its mucous lining is exceedingly extensive.
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