CHARACTERS OF FISHES. 341 



124, d) are usually much larger and thicker than the ordinary 

 scales, and though they are often articulated to one another by 

 special processes, they only rarely overlap. IV. Placoid scales, 

 consisting of detached bony grains, tubercles, or plates, of 

 which the latter are not uncommonly armed with spines 

 (fig. 124, c). 



In most fishes there is also to be observed a line of peculiar 

 scales, forming what is called the " lateral line." Each of the 

 scales in this line is perforated by a tube leading down to a 

 longitudinal canal which runs along the side of the body, and 

 is connected with cavities in the head. The function of this 

 singular system has been ordinarily believed to be that of se- 

 creting the mucus with which the surface of the body is covered; 

 but it seems to be more probably sensory in function, and to 

 be connected with the sense of touch. 



As regards their true osseous system or endoskeleton, Fishes 

 vary very widely. In the Lancelet there can hardly be said to 

 be any skeleton, the spinal cord being simply supported by 

 the gelatinous notochord, which remains throughout life. In 

 others the skeleton remains permanently cartilaginous \ in 

 others it is partially cartilaginous and partially ossified ; and, 

 lastly, in most modern fishes it is entirely ossified or converted 

 into bone. Taking a bony fish (fig. 125) as in this respect a 

 typical example of the class, the following are the chief points 

 in the osteology of a fish which require notice : 



, 



bones; it Interspinous bones; rRibs; j Spinous processes of vertebrae ; h Haemal 

 processes of vertebras. 



The vertebral column in a bony fish consists of vertebras, 

 which are hollow at both ends, or biconcave, and are techni- 



