20 



The Exterior of the Fish 



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 



Fig. 12. — Rock Hind or Cabra Mora of the West Indies, Epinephelus adscensionis 

 (Osbook). Famih' Serranidcc. 



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, ma}' 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 Cycloid Scales. — Nomially formed scales are 

 rounded in outline, marked by fine concentric, rings, an(l' crossed 

 on the inner side by a few strong radiatmg ridges and folds 



