310 THE SEA SHOEE 



thoroughly embedded in the slimy skin as not to be discovered with- 

 out the aid of a microscope. When the scales have unbroken edges 

 and overlap one another they are said to be cycloid, but when the 

 projecting edges are toothed or serrated, giving a roughness to the 

 touch, they are described as ctenoid. Sometimes the scales are 

 modified into bony plates or little isolated bony granules, and in 

 either case they are practically identical in structure with teeth, 

 consisting as they do of dentine, capped with a little harder sub- 

 stance resembling enamel. 



We often observe a row of scales, of a different nature from 

 those covering the body generally, running along each side of a fish 

 from near the eye to the end of the tail ; and these constitute what 

 is called the lateral Une. If we examine these scales closely, we 

 observe that each one is pierced by a hole that communicates with 

 a little sac beneath containing a gelatinous material, and in which 



Fio. 227. THE PILCHARD 

 1. Dorsal fin. 2. Pectoral fin. 3. Pelvic fin. 4. Ventral or anal fin. 5. Candal fin. 



a nerve tendril terminates. The presence of the nerve filament 

 evidently denotes that the scales in question, with the little sacs 

 beneath them, are organs connected with sensation, and it is also 

 believed that they have something to do with the secretion of the 

 slimy mucus that covers the scales of the body. 



The mouth of a fish is generally situated on the extreme front 

 of the head, but occasionally, as in the sharks and rays, quite on the 

 under side. If it contains a tongue at all, this organ is of small 

 size and simple structure ; thus it is highly probable that the sense 

 of taste is very feeble in these animals, and this is just what one 

 might expect when one remembers that fishes never retain their food 

 in the mouth for any length of time, but simply bolt it without any 

 attempt at mastication. 



The arrangement and nature of the teeth are very variable. 

 Often they are developed on the membrane of the mouth only, in 

 which case they are generally renewed as fast as they are worn 



