462 MANUAL OF ZOOLOGY. 



having their posterior margins fringed with spines, or cut into 

 comb-like projections. III. Ganoid scales, composed of an 

 inferior layer of bone, covered by a superficial layer of hard 

 polished enamel (the so-called "ganoine"). These scales 

 (fig. 247, e) 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 or dentinal grains, 

 tubercles, or plates, of which the latter are not uncommonly 

 armed with spines (fig. 247, c and d). 



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 

 secreting the mucus with which the surface of the body is cov- 

 ered ; but this is certainly erroneous, and 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 persists 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. 248) 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 technically said 

 to be " amphiccelous." The cup-like margins of the vertebral 

 bodies are united -by ligaments, and the cavities formed between 

 contiguous vertebrae are filled with the gelatinous remains of 

 the notochord. This elastic gelatinous substance acts as a 

 kind of ball-and-socket joint between the bodies of the verte- 

 brae, thus giving the whole spine the extreme mobility which is 

 requisite for animals living in a watery medium. The ossifi- 

 cation of the vertebrae is often much more imperfect than the 

 above, but in no case except that of the Bony Pike (Lepidos- 

 teus) is ossification carried to a greater extent than this. In 



* The so-called "bone" of the skeleton of Fishes is only occasionally 

 true osseous tissue. In a great many instances it is a homogeneous or tubu- 

 lar, bone-like substance, or it may resemble genuine dentine. 



