EXOSKELETON OF FISHES. 643 



same way as the shelly covering of a mollusk ; and by maceration the 

 different layers may readily be separated, the smallest and most super- 

 ficial being of course the first formed, while the largest and most recent 

 are those nearest to the surface of the living skin : as far as relates to 

 the mode of growth, therefore, there is the strictest analogy between 

 the scale of a fish and shell. Various are the forms under which these 

 scales present themselves to the ichthyologist : sometimes, as in the Eel, 

 they are thinly scattered over the surface of a thick and slimy cutis ; 

 more generally they form a close and compact imbricated mail ; in the 

 Pipe-fishes (Syngnaihidce) the whole body is covered with a strong 

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

 fishes (Ostracionidce) the integument is converted into a strong box made 

 up of polygonal pieces anchylosed together, so that the tail and fins alone 

 remain moveable. 



(1751.) The Sturgeon is covered with broad shield-like plates. The 

 skin of the Shark is densely studded with minute sharp spines of almost 

 crystalline hardness ; and in many Skates, as in the Thornback, similar 

 cuticular appendages, but of more considerable dimensions, are distri- 

 buted over the back and tail, forming very efficient defensive weapons. 



(1752.) But cutaneous spines, although while in a rudimentary con- 

 dition they are obviously mere extraordinary developments 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 demon- 

 strate 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 endosTceleton properly so called. 



(1753.) Let us examine this important subject with a little attention, 

 and we shall soon perceive how closely the endoskeleton and the exo- 

 skeleton 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 cuticular appendages, 

 secreted in the same manner as scales from the surface of a vascular 

 pulp; but in the Fireflare (Trygon pastinaca), 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 in- 

 ternally, we might be tempted to regard this appendage as a process 

 derived from the endoslceleton. 



(1754.) The spines of the common Stickleback (Gasterosteus) are 

 indubitable derivations from the cuticle; but they are fastened by 

 moveable articulations to the sides of the body, and are raised or de- 



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