The Exterior of the Fish 19 
with age, and is sometimes larger also in the male sex. The 
development of the fins often varies a good deal in some fishes 
with age, old fishes and male fishes having higher fins when 
Fic. 12.—Rock Hind or Cabra Mora of the West Indies, Epinephelus adscensionis 
(Osbeck), Family Serranide. 
such differences exist. These variations are soon understood by 
the student of fishes and cause little doubt or confusion in the 
study of fishes. 
The Scales, or Exoskeleton.—The surface of the fish may be 
naked as in the catfish, or it may be covered with scales, prickles, 
shagreen, or bony plates. The hard covering of the skin, when 
present, is known as the exoskeleton, or outer skeleton. In the 
fish, the exoskeleton, whatever form it may assume, may be 
held to consist of modified scales, and this is usually obviously 
the case. The skin of the fish may be thick or thin, bony, 
horny, leathery, or papery, or it may have almost any inter- 
mediate character. When protected by scales the skin is usually 
thin and tender; when unprotected it may be ossified, as in the 
sea-horse; horny, as in the headfish; leathery, as in the catfish; 
or it may, as in the sea-snails, form a loose scarf readily de- 
tachable from the muscles below. 
The scales themselves may be broadly classified as ctenoid, 
cycloid, placoid, ganoid, or prickly. 
Ctenoid and Cyclotd Scales—Normally formed scales are 
rounded in outline, marked by fine concentric rings, and crossed 
on the inner side by a few strong radiating ridges and folds. 
