Fishes . may indicate the choiceof snails and crabs as food. Two ormore of these different types may beThe knife-like teeth of the sharks areprogressively shed, new ones being constantly formed on theinner margins of the jaw, so that the teeth are marching to belost over the edge of the jaw as soon as each has fulfilled itsfunction. In general the more distinctly a species is a fish-eater, the sharper are the teeth. Usually fishes show little dis-crimination in their choice of food; often they devour the youngof their own species as readily as any other. The digestiveprocess is rapid, and most
Fishes . may indicate the choiceof snails and crabs as food. Two ormore of these different types may beThe knife-like teeth of the sharks areprogressively shed, new ones being constantly formed on theinner margins of the jaw, so that the teeth are marching to belost over the edge of the jaw as soon as each has fulfilled itsfunction. In general the more distinctly a species is a fish-eater, the sharper are the teeth. Usually fishes show little dis-crimination in their choice of food; often they devour the youngof their own species as readily as any other. The digestiveprocess is rapid, and most fishes rapidly increase in size in theprocess of development. When food ceases to be abundant thenshes grow more slowly. For this reason the same species willgrow to a larger size in large streams than in small ones, in lakesthan in brooks. In most cases there is no absolute limit togrowth, the species growing as long as it lives. But while somespecies endure many years, others are certainly very short-. FiG. 18.—Jaws of a Parrot-fish, Sparisoma aurofrenutum(Val.). Cuba. found in the same fish. The Dissection of the Fish lived, and some may be even annual, dying after spawning, per-haps at the end of the first season. Teeth are wholly absent in several groups of fishes. Theyare, however, usually present on the premaxillary, dentary, andpharyngeal bones. In the higher forms, the vomer, palatines,and gill-rakers are rarely without teeth, and in many cases thepterygoids, sphenoids, and the bones of the tongue are similarlyarmed. Xo salivary glands or palatine velum are developed in tongue is always bony or gristly and immovable. Some-times taste-buds are developed on it, and sometimes these arefound on the barbels outside the mouth. The Alimentary Canal.—The mouth-cavity opens through thepharynx between the upper and lower pharyngeal bones into the
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