46 



FISHES. 



Fig. 10.— Cycloid scale of 

 Gadopsis marmoratus (magn.) 



frequently naked tlian scaly. All fishes provided with 

 electric organs, the majority of Eels, and the Lampreys, are 

 naked. Scales of fishes are very different from those of 

 Reptiles ; the latter being merely folds of the cutis, wliilst 

 the scales of fishes are distinct horny elements, developed in 

 grooves or pockets of the skin, like hairs, nails, or feathers. 

 Very small or rudimentary scales are 

 extremely thin, homogeneous in struc- 

 ture, and more or less imbedded in 

 the sldn, and do not cover each other. 

 When more developed, they are im- 

 bricated (arranged in the manner of 

 tiles), with the posterior part extruded 

 and free, the surface of the anterior portion being usually 

 covered by the skin to a greater or less extent. On their 



surface (Tigs. 10 and 11) may 

 be observed a very fine striation 

 concentric and parallel to the 

 margin, and coarser striaj radi- 

 ating from a central point to- 

 wards the hind margin. Scales 

 without a covering of enamel, 

 with an entire (not denticu- 

 lated) posterior margin, and 

 wdth a concentric striation, are 

 called Cycloid scales. Ctenoid 

 scales (Figs. 12-15) are gene- 

 rally thicker, and provided with spinous teeth on the posterior 

 edges of the layers of which the scale consists. In some 

 species only the layer nearest to the margin is provided with 

 denticulatious (Fig. 14). Scales, the free surface of which is 

 spiny, and which have no denticulation on the margin, have 

 been termed Sparoid scales ; but their distinction from ctenoid 

 scales is by no means sharp, and there are even intermediate 



Fig. 11. — Cycloid scale of Scopelus 

 resplendens (magn.) 



