The Exterior of the Fish 21 



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 papillae, 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 FIQ 13 _ Scales of 

 some sculpins are not placoid, but are re- Acanthoessus bronni 

 duced or modified ctenoid scales, scales nar- (Agassiz). (After 

 rowed and reduced to prickles. 



Bony and Prickly Scales. Bony and prickly scales are 

 found in great variety, and scarcely admit of description or 

 classification. In general, prickly points on the skin are modifi- 

 cations of ctenoid scales. Ganoid scales are thickened and cov- 

 ered with bony enamel, much like that seen in teeth, otherwise 





