PISCES FISHES. 507 



is the strictest analogy between the scale of a fish and shell. Va- 

 rious are the forms under which these scales present themselves to 

 the icthyologist : sometimes, as in the Eel, they are thinly scat- 

 tered over the surface of a thick and slimy cutis, more generally 

 they form a close and compact imbricated mail ; in the Pipe- 

 fishes (Syngnathidee) the whole body is covered with a strong 

 armour composed of broad and thick calcareous plates ; and in the 

 Coffin-fishes (OstracionidtE) the integument is converted into a 

 strong box made up of polygonal pieces anchylosed together, so 

 that the tail and fins alone remain moveable. 



The Sturgeon is covered with broad shield-like plates. The 

 skin of the Sharks is densely studded with minute sharp spines of 

 almost crystalline hardness ; and in many Skates, as in the Thorn- 

 back, similar cuticular appendages, but of more considerable 

 dimensions, are distributed over the back and tail, forming very 

 efficient defensive weapons. 



But cutaneous spines, although while in a rudimentary condi- 

 tion they are obviously mere extraordinary developements of scales, 

 may occasionally become of sufficient size and importance to make 

 them convertible to various unexpected uses ; and when thus 

 exaggerated in their dimensions, and appropriated to distinct offices, 

 they assume so much of the character of true bone, that it is no 

 longer easy to demonstrate their real nature, more especially as 

 they then become in many cases really articulated by means of very 

 perfect joints with different pieces of the endoskeleton properly so 

 called. 



Let us examine this important subject with a little attention, 

 and we shall soon perceive how closely the endoskeleton and the 

 exoskeleton may become connected, not to say interchangeable, with 

 each other. There is no possibility of mistaking the spines and 

 tubercles upon the back of a common Skate for anything but cuti- 

 cular appendages secreted in the same manner as scales from the 

 surface of a vascular pulp ; but in the Fire Flaire (Trygon pasti- 

 naca), where, instead of the scattered hooks of the former species, 

 we find a single sharp and serrated spine projecting like a bayonet 

 from the upper surface of the root of the tail, the analogy between 

 this formidable and bone-like organ and an epidermic structure 

 becomes apparently more remote, and, did we not know that the 

 fish possessing such a weapon had no ossified bones internally, we 

 might be tempted to regard this appendage as a process derived 

 from the endoskeleton. 



