FISHES. 



257 



the whole structure of the living temple in its course, 

 it takes its return j ourney as we at first described it. V 



The lowest Class of the great Vertebrate Division is 

 that of Fishes. They are distinguished by the simplicity 

 of their outline, by their respiring by gills instead of 

 lungs, by the enormous development of their hyoid appa- 

 ratus, by their cold blood, by the modification of their 

 limbs into fins, and by the possession of accessory organs 

 of the same kind, especially the tail-fin, which is their 

 grand instrument of locomotion. All these characteristics 

 are, more or less obviously, dependent on the great fact 

 of their aquatic life. 



We have already adverted to the existence among the 

 Mollusoa of the rudiments of an internal skeleton, by 

 which that great division overlaps, so to speak, the pre- 

 sent. On the other band, we find in most Fishes rem- 

 nants of the external skeleton neither few nor unimpor- 

 tant, by which they manifest their affinity with the crea- 

 tures r^elow them. The scales of the majority of Fishes, 

 the bony plates which we see in the Trunk-fish {Ostracion) 

 and in the " Tittlebat," which every truant schoolboy 

 knows, the recurved spinous tubercles with which the 

 Thornback's skin is studded, and the opercular bones or 

 plates that cover the gills, — what are all these but portions 

 of an external skeleton, in no way belonging to that series 

 of bones which belongs to the fish as a vertebrate animal ? 

 The rays of the fins which are not limbs (as the dorsal, 

 the anal, and the caudat), and the blade-like bones pene- 

 trating the flesh to which these are jointed, must also come 

 into the same category. 



The scales which form the covering of most fishes are 



