iS A Guide to the Zoological Collections 



The pectoral fins may be altogether absent, as they are in some eels. 

 However they are far more constant than the ventrals, and although the 

 height at which they stand varies a little, their position is always constant 

 immediately behind the gill-opening. Their extreme of development is 

 found in the Flying-fishes. In the Paradise- fishes (Polynemus : " Topsi") 

 some of their rays are of prodigious length and form organs of touch. The 

 same thing is seen in Bathypterois Guentheri a fish that lives in great depths 

 of the ocean where little sunlight penetrates and where therefore eyes, being 

 probably useless, are liable to degenerate. Degeneration of the eyes lias 

 occurred in Bathypterois and, in compensation, some of the rays of the 

 pectoral fins have become long " feelers," by means of which the animal 

 probably feels its way in the perpetual gloom, in much the same way as a 

 blind man feels his way with a stick. 



The ventral fins are far more variable in position than]the^ pectorals and 

 are usually smaller. In some fishes, for instance in all eels, they are 

 altogether absent. In others, as in the whole Order of Physostomes they 

 are, when present, placed far back on the belly. In others, as in the Perch 

 we are considering, they are placed far forwards under, or just behind, the 

 pectorals. In others, as in the Cod-fishes they are placed on the throat, well 

 in front of the pectorals. 



Although the paired fins of Fishes correspond with 

 the limbs of higher Vertebrates, and although they are 

 of some use as motors, they are not the principal means 

 of locomotion ; for locomotion in most fishes is effected 

 by the pliant muscular tail and the caudal fin. 



In most fishes the SKIN, as in the specimen of Sea- 

 perch that we are examining, is covered with scales, 

 which are often imbricate. Scales are of four principal 

 kinds, namely : — (i) Cycloid scales, which are thin horny 

 plates with a thin smooth edge; (2) Ctenoid scales, 

 which are thicker horny plates with sharp spines on the 

 hinder edge and often also on the exposed surface; (3) 

 Ganoid scales, which are hard bony plates covered with 

 a layer of still harder enamel : they do not overlap, but 



