. Fishes. Fishes. 20 The Exterior of the Fish They usually cover the body more or less evenly and are imbri- cated like shingles on a roof, the free edge being turned back- ward. Such normal scales are of two types, ctenoid or cycloid. Ctenoid scales have a comb-edge of fine prickles or cilia; cycloid scales have the edges smooth. These two types are not very different, and the one readily passes into the other, both being sometimes seen on different parts of the same fish. In general, however, the more primitive representatives of the typical fishes, those with abdominal ventrals and without
. Fishes. Fishes. 20 The Exterior of the Fish They usually cover the body more or less evenly and are imbri- cated like shingles on a roof, the free edge being turned back- ward. Such normal scales are of two types, ctenoid or cycloid. Ctenoid scales have a comb-edge of fine prickles or cilia; cycloid scales have the edges smooth. These two types are not very different, and the one readily passes into the other, both being sometimes seen on different parts of the same fish. In general, however, the more primitive representatives of the typical fishes, those with abdominal ventrals and without spines in the fins, have cycloid or smooth scales. Examples are the salmon, herring, minnow, and carp. Some of the more specialized spiny-rayed fishes, as the parrot-fishes, have, however, scales equally smooth, although somewhat different in structure. Sometimes, as in the eel, the cycloid scales may be reduced to mere rudiments buried in the skin. Ctenoid scales are beset on the free edge by little prickles or points, sometimes rising to the rank of spines, at other times soft and scarcely noticeable, when they are known as ciliate or eyelash-like. Such scales are possessed in general by the more specialized types of bony fishes, as the perch and bass, those with thoracic ventrals and spines in the fins. Placoid Scales.—Placoid scales are ossified papilla, minute, enamelled, and close-set, forming a fine shagreen. These are characteristic of the sharks, and in the most primitive sharks the teeth are evidently modifications of these primitive structures. Some other fishes have scales which appear shagreen-like to sight and feeling, but only the sharks have the peculiar structure to which Agassiz gave the name of placoid. The rough prickles of the filefishes and some sculpins are not placoid, but are re- duced or modified ctenoid scales, scales nar- rowed and reduced to prickles. Bony and Prickly Scales.—Bony and. Fig. 13.—Scales of A canthoessus bronni (.Agassiz). (After
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