Page:Natural History, Fishes.djvu/163

Rh close together; the caudal is large, forked, or crescent-shaped; the upper division sometimes more developed than the lower; the head is obtuse and high, the profile approaching to vertical.

The fishes of this genus are reckoned among that small number of the Class which feed entirely on vegetable substances: sea-weeds, and such like marine vegetation, that may generally be found fringing the submerged rocks in the tropical seas, afford them an always abundant repast. One result of this diet is, that the flesh, though commonly eaten, has a peculiar flavour, disagreeable to many persons. The intestinal canal, as usual in herbivorous animals, is long and complicated.

We must consider the caudal lancets of these fishes as defensive, rather than offensive, weapons, analogous to the horns of the ruminant Mammalia. They are highly curious; each consists of a curved, flattened spine, lodged in a membranous sheath in the side of the tail, and ordinarily concealed; but capable of being partially elevated, as on a hinge, at the will of the animal, when its acute point and keen edge are found to point backward. They are exceedingly firm in texture, of a hard crystalline substance; and being violently jerked from side to side, by the action of the powerful lateral muscles, they doubtless constitute formidable weapons, and enable the browsing fish effectively to repel any carnivorous foe that may be inclined to attack him in the rear.

We shall illustrate the genus by a species which the inhabitants of Jamaica distinguish as the White Doctor-fish (Acanthurus cæruleus,