CHARACTERS OF FISHES. 363 



" ganoine"). These scales (fig. 132, 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. 132, ^r). - 



In most fi.shes 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. 133) 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 : — 



The vertebral column in a bony fish consists of vertebrae, 

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

 cally said to be "amphicoelous." The cup-like margins of the 

 vertebral bodies are united by ligaments, and the cavities 

 formed between contiguous vertebrae are filled with the gela- 

 tinous remains of the notochord. This elastic gelatinous sub- 

 stance acts as a kind of ball-and-socket joint between the bodies 

 of the vertebrae, thus giving the whole spine the extreme mo- 

 bility which is requisite for animals living in a watery medium. 

 The ossification of the vertebrae is often much more imperfect 

 than the above, but in no case except that of the Bony Pike 

 {Lepidosteus) is ossification carried to a greater extent than 

 this. In this fish, however, the vertebral column is composed 

 of" opisthocoelous" vertebra — that is, of vertebree the bodies of 

 which are concave behind and convex in front The entire 

 spinal column is divisible into not more than two distinct re- 

 gions, an abdominal and a caudal region. The abdominal ver- 

 tebrae possess a superior or neural arch (through which passes 

 the spinal cord), a superior spinous process (neural spine), and 

 two transverse processes to which the ribs are usually attached. 

 IT 



