FISHES. 



253 



degrees, of the calcareous element, it becomes hard, solid, 

 and inflexible, and we call it bone. 



Now, as in the highest forms among the Mollusca we 

 saw the external skeleton of shell gradually vanishing, 

 and traces of an internal skeleton of cartilage appearing, 

 (as the cranial ring, or skull, and the fin-plates, of the 

 Cuttle-fishes) — so in the most rudimentary of the Fishes; 

 as the Lamprey, and that curious creature the Sea-hag 

 (Myxine), and, more markedly still, in the dubious Lance- 

 let (Amphioxus), the spinal column, instead of forming a 

 series of distinct bones, is an undivided rod of cartilage ; 

 in the latter two instances horny, flexible, and bearing the 

 closest resemblance to the " pen" of the common Squid. 



Perhaps it will be well to examine the nature of this 

 distinctive skeleton as we find it in its normal development. 

 Its most important element is the spinal column, which con- 



Vertebrse of a fish. 



a. Lateral arpect. b. Front aspect 



Bists of a linear series of pieces attached to each other, and 



