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 
Fig. 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. ain 
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 
