506 PISCES FISHES. 



without dissection, might seem to be constructed on the same prin- 

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

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

 uninterruptedly 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. 



(541.) If in the highest HETEROGANGLIATA we found, that in 

 addition to the tegumentary skeleton, or shelly covering, so exten- 

 sively 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 inter- 

 nally, 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 fishes the coexistence 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 exoske- 

 leton, which, as we shall soon perceive, forms no unimportant part 

 of the anatomy of the class under consideration. 



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 admirably 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 exte- 

 rior and beautiful arrangement they ensure the least possible resist- 

 ance from the surrounding medium as the fish glides along. 



(54.) Examined separately, each scale is found to be partially 

 embedded 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 

 superimposed laminae of horny matter secreted by the cutis, pre- 

 cisely in the same way as the shelly covering of a mollusk, and 

 by maceration the different layers may readily be separated, the 

 smallest and most superficial 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 



