SKELETON OF FISHES. 3 



colours with which these animals are adorned, astonish us by 

 their variety and splendour. The silvery matter, which frequently 

 gives to them so beautiful a metallic splendour, is secreted by the 

 skin, and is composed of a number of small polished laminae. 

 Along each side of the body in most Fishes there is a single series 

 of peculiar scales, eacli bearing a minute tube. This series of scales 

 is commonly known as the lateral line, and its differences often 

 furnish valuable characters for the determination of the genera and 

 species of Fishes. The object of this curious arrangement is not 

 yet well made out. The little tubes open into a canal which runs 

 along beneath the lateral line ; this has generally been regarded 

 as a glandular organ furnishing that slimy secretion with which 

 the skins of all Fishes are covered. By modern Zoologists, how- 

 ever, this slimy matter is considered to represent the outer coat 

 of the epidermis ; and it appears that the lateral canals communi- 

 cate with a curious arrangement of tubes situated in the head, and 

 furnished with nerves, the use of which has not been ascertained. 

 539. The skeleton of Fishes is usually bony ; but in many 

 cases, as in the Ray and the Shark, it remains permanentl}' in a 

 fibre-cartilaginous or cartilaginous state ; and in some this frame- 

 work possesses even still less solidity, and remains perfectly mem- 

 branous : certain Lampreys are in this condition, and thus form 

 a transition between the Vertebrated and Invertebrated animals. 



FIG. 346. SKELETON OF THE PERCH. 



The bones never have any medullary canal ; and the cartilage 

 which constitutes their foundation is not the same, as that of the 



B 2 



