490 PISCES FISHES. 



offers an appearance sketched in the adjoined figure (fig. 220, A). 

 In a more advanced form of a fish's skeleton, as for example in the 



Sturgeon, these ossified rings are 



P j , , & ., Fig. wo. 



found to have enlarged consider- 



ably, and penetrate still more * - - *- - * i 



deeply into the cartilaginous mass 

 (^g.220, B). As the bony rings 

 thus developed approximate the 

 centre, it becomes more and more 

 evident that they represent the 

 bodies of so many vertebrae ; but 

 even in the majority of fishes the 

 central part remains permanently 

 unossified ; so that a cartilaginous 

 axis traverses the vertebral co- 

 lumn from one end to the other 

 (Jig. 220, c), and it is not usual 



to find the central aperture perfectly obliterated, as delineated in 

 the fourth sketch, D. 



(524.) Fishes, being continually resident in an element nearly 

 of the same specific gravity as their own bodies, require little firm- 

 ness or solidity in the construction of their spinal column : a free 

 and unfettered power of flexion in certain directions so as to 

 permit an ample sweep of their expanded tail, which forms the 

 principal agent in propelling them forwards, is far more essential 

 to their habits. Thus the cartilaginous spine of the feeble Lam- 

 prey is sufficient for all needful purposes ; and even in the most 

 perfectly ossified fishes, from the manner in which the vertebrae 

 are united to each other, the greatest possible flexibility is ensured. 

 The body of each vertebra presents two conical cups, the apices 

 of which are nearly or quite continuous ; the margin of each cup- 

 like depression is united by elastic ligament to the corresponding 

 margin of the contiguous vertebra, and thus between the bodies of 

 each pair of vertebrae a wide cavity is formed (D), which is filled 

 up with a semi-gelatinous substance; so that, by this beautiful con- 

 trivance, the mobility of the whole chain is abundantly provided 

 for. 



(525.) There are only two kinds of vertebrae recognizable in 

 the skeleton of a fish, viz. the abdominal and the caudal. The 

 abdominal vertebrae support the ribs, for in these animals the ribs 

 do not constitute a thorax, or contain any of the viscera called 



