642 PISCES. 



system is therefore necessarily changed to meet these altered circum- 

 stances : the muscles of the trunk, which in osseous fishes formed the 

 great agents in locomotion, become now of secondary importance ; while 

 those of the pectoral fins, so feebly developed in the Perch, are massive 

 and powerful in proportion to the unwieldy size of the anterior extremities. 

 Another peculiarity in the skeleton of the Chondropterygii is observable 

 in the construction of the caudal fin, which, even in the Sturgeon and the 

 Shark, notwithstanding the importance which this organ still maintains 

 in those genera as an instrument of locomotion, begins to differ very re- 

 markably from the tail of an osseous fish. It is true that it still exhibits 

 great expansion in a vertical direction, and to a superficial observer, if 

 examined without dissection, might seem to be constructed on the same 

 principles ; but, on examining the skeleton of one of these cartilaginous 

 fishes, it will be found that the vertebral column is continued uninter- 

 ruptedly into the upper half of the generally furcate tail, whilst the 

 lower division of the caudal fin is entirely made up of supplementary 

 rays, appended to the inferior aspect of the caudal vertebrae. Possessing 

 this form of the tail, the transition is by no means abrupt from these 

 highly-organized fishes to the Saurian reptiles, with which, as we shall 

 afterwards see, they exhibit many remarkable affinities. 



(1748.) If in the highest HETEKOGANGLIATA we found that, in addi- 

 tion to the tegumentary skeleton, or shelly covering, so extensively met 

 with among the Mollusca, the first appearances of an internal osseous 

 system became recognizable, we are not on that account to imagine that, 

 as soon as bones become developed internally, the cuticular secretions 

 hitherto denominated shell at once disappear, but, on the contrary, must 

 be prepared to expect that, in some form or other, calcareous armour 

 deposited by the skin should still be met with. In Pishes, the coexist- 

 ence of an internal and of an external skeleton is undeniable; and 

 having already described the former, which has been aptly enough 

 called the endoskeleton, it remains for us in the next place to examine 

 the latter or exoskeleton, which, as we shall soon perceive, forms no un- 

 important part of the anatomy of the class under consideration. 



(1749.) The most usual form of the cuticular covering of Fishes is 

 that of imbricated scales, with which the whole exterior of the body is 

 compactly encased, as in a suit of armour. Such an investment is ad- 

 mirably adapted to their habits and economy. The dense and corneous 

 texture of the scales, impermeable to water, defends their soft bodies 

 from maceration, while, from their smooth, polished exterior and beau- 

 tiful arrangement, they ensure the least possible resistance from the 

 surrounding medium as the fish glides along. 



(1750.) Examined separately, each scale is found to be partially im- 

 bedded in a minute fold of the living and vascular cutis, to which its 

 under surface is adherent. Every scale is, in fact, made up of super- 

 imposed laminae of horny matter secreted by the cutis, precisely in the 



