46 



FISHES. 



Fig. 10. — Cycloid scale of 

 Gadopsis marmoratus (magn.) 



frequently naked than scaly. AH fishes provided with 

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

 naked. Scales of fishes are very different from those of 

 Eeptiles; the latter being merely folds of the cutis, whilst 

 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 skin, 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 

 ^gs^^^,^ surface (Tigs. 10 and 11) may 



. V be observed a very fine striation 



I ^ ^ "~^ ,j. concentric and parallel to the 



margin, and coarser strise radi- 

 ating from a central point to- 

 wards the hind margin. Scales 

 without a covering of enamel, 

 V^ with an entire (not denticu- 



lated) posterior margin, and 

 with 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 

 denticulations (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 



i' 



Fig. 11. — Cycloid scale of Scopelus 

 resplendens (magn.) 



