IRISHES'. 307 



Instead of imbricated scales, some fishes are protected by 

 osseous plates, covered, like the scales, by the cuticle, and 

 presenting an even surface. Among some of the sharks, 

 as the Squalus dcahtTiias, instead of scales there are flat, 

 bent, bristly laminae ; and in the remora there are hard, 

 rough tubercles. These osseous plates in the sturgeon, re- 

 semble in shape the shell of a limpet. 



The naturalist employs the appearances exhibited by 

 the form, surface and size of the scales, as a character in the 

 discrimination of nearly allied species, although the dispo- 

 sition of the longitudinal and the transverse rays, together 

 with the condition of the margin, would furnish more per- 

 manent marks. The scales, in the description of a fish, are 

 likewise considered in regard to their adhesion to the skin. 

 Thus some scales, which adhere but slightly, are said to be 

 deciduous ; while others, which cannot be rubbed off but 

 with difficulty, are termed tenacious or adhesive. 



Besides the scales many fishes are furnished with spin- 

 CMS processes. These sometimes accompany the fins ; while 

 in other instances they appear as the armature of the head 

 and cheeks. They appear to be of the same consistence 

 and composition as horn. Those found on the head are in 

 general fixed ; but those connected with the fins are moved 

 by peculiar muscles. These organs may be considered as 

 defensive weapons, and act, in some instances, not merely 

 by their form and consistence, but by some venomous secre- 

 tion by which they are covered *, 



* Thus the common weever (Trachinus draco) inflicts a wound with the 

 spines of the first dorsal fin, often followed by violent burning pains, inflam- 

 mation, and swellings; so that the fishermen are in the practice of cutting 

 off the offensive organ before they bring the fish to market. The spines of 

 the Squalus acantkias, or piked dog-fish, and Doras carinatus, are likewise 

 considered by fishermen as capable of inflicting a dangerous wound. 



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