FISHES. 229 



spines. The common Pei-cli suii|)lies a good example of these 

 scales. 



3. Placoid scales (f, fl). — These are detached bony grains, tuljercles, 

 or plates, scattei'ed 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 obseived a line of jieculiar scales, 

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

 line is perfoi-ated by a minute tube, which leads into a longitudinal 

 canal, supplied with si}ecial nerves ; and the whole apparatus has 

 apparently a sensory function. 



As regards the true internal skeleton, fishes differ very widely 

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

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

 fish — the Laucelet — thei'e can hardly be said to be any true skeleton, 

 the vertebral column being represented permanently by the semi- 

 gelatinous notochord (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 com]iletel_y 

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

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

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

 united together by ligaments. The cavities formed by the ap])Osi- 

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

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

 socket joint between the vertebra^, thus giving the whole spine the 

 extrenre 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 

 vertebr;e possess a superior or neural arch, for the passage of the 

 spinal cord, a superior spinous process, and two transverse pi'ocesses 

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

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

 mentioned, they give ofi' an inferior or Juemal arch, helow the body 

 of the vertebrte, and they carry inferior spi/wus jjrocesses (fig. 16.5, 

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

 bodies of the abdominal vertebr;ie (fig. 1G5, r) ; and they do not 

 10 



