PISHES. 
257 
the whole structure of the living temple in its course, 
it takes its return journey as we at first described it. 
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 
whicli that great division overlaps, so to speak, the pre- 
sent. On the other hand, 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 below them. The scales of the majority of Fishes, 
the bony plates whicli 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 whicli belongs to the fish as a vertebrate animal 1 
The rays of the fins which aro not limbs (as the dorsal, 
the anal, and the caudal ), and the blade-like bones pene- 
trating the flesh to which these aro jointed, must also come 
into the same category. 
The scales which form, the covering of most fishes are 
K 
