FISHES. 229 



spines. The common Perch supplies a good example of these 



3. Placoid scales (c, d). — These are detached bony grains, tubercles, 

 or plates, scattered through the skin, and sometimes armed with 

 projecting spines, these latter being usually composed of a substance 

 allied to dentine. 



4. Ganoid scales (e), composed of a layer of true bone, covered by 

 a layer of hard polished enamel. — These scales are usually much 

 thicker and larger than the ordinary scales ; they are often oblong 

 or rhomboidal in shape ; they are often connected together by little 

 processes; and they generally are in contact by their edges, but 

 rarely overlap one another. 



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 of this 

 line is perforated by a minute tube, which leads into a longitudinal 

 canal, supplied with special nerves ; and the whole apparatus has 

 apparently a sensory function. 



As regards the true internal skeleton, fishes diifer very widely 

 from one another, but the skeleton is so complicated that only a 

 few of the most important points can be mentioned here. In one 

 fish — the.Lancelet — there can hardly be said to be any true skeleton, 

 the vertebral column being represented permanently by the semi- 

 gelatinous notoohord (fig. 171). In others, such as the Lampreys, 

 Sturgeons, and Rays, the skeleton remains permanently in the con- 

 dition of gristle (cartilage) ; in others, it is partially cartilaginous and 

 partially ossified ; and lastly, in most modern fishes it is completely 

 converted into bone. The vertebral column in a Bony Fish consists 

 of a number of vertebr« which are hollow or cup-shaped at both 

 ends (biconcave or " amphicoelous "), the cup-like margins being 

 united together by ligaments. The cavities formed by the apposi- 

 tion of the vertebrae are filled with the gelatinous remains of the 

 notochord. This gelatinous elastic substance acts as a ball-and- 

 socket joint between the vertebrte, thus giving the whole spine the 

 extreme flexibility which is essential to animals living in a watery 

 medium. The entire spinal column is divisible into no more than ' 

 two distinct regions, an abdominal and a caudal. The abdominal 

 vertebrae possess a superior or neural arch, for the passage of the 

 spinal cord, a superior spinous process, and two transverse processes 

 to which the ribs are attached. The caudal vertebrae have no marked 

 transverse processes ; but, in addition to the other parts already 

 mentioned, they give off an inferior or hcemal arch, below the body 

 of the vertebrae, and they carry inferior spinous processes (fig. 16.5, 

 h). The ribs are attached to the transverse processes or to the 

 bodies of the abdominal vertebrae (fig. 165, r) ; and they do not 

 11 



