54 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



latter is furnished with two or three pairs of jaws which open sideways, that 

 of the former has never more than one pair of jaws, which are placed one 

 above or before the other; and these jaws are usually armed with teeth, which 

 are very analogous in their structure to bone. 



7. General characters of Fishes. 



26. The Vertebrata are subdivided into classes, principally according to 

 their mode of performing the functions of respiration and reproduction. Thus, 

 FISHES are at once separated from all other groups, by the circumstances of 

 their being adapted, like the aquatic Invertebrata, to aerate their blood by 

 gills ; and being hence enabled to inhabit water during their whole lives, 

 without the necessity of coming to the surface to breathe. The low amount 

 of their respiration prevents their bodies from ever attaining a temperature 

 much above that of the surrounding medium ; hence they are spoken of as 

 cold-blooded. Further, they are oviparous ; an ovum or egg being deposited 

 by the parent, from which, in due time, the young makes its way ; or if, as 

 sometimes happens, the ovum is retained within the body of the parent until 

 it is hatched, the young animal, though produced alive, is not subsequently 

 dependent upon its parent for support. In many respects, the organization 

 of Fishes is not much advanced beyond that of the higher Mollusca. Their 

 respiratory apparatus has the same character; and the organs by which the 

 blood is depurated of its superfluous azote, rather correspond with the tem- 

 porary Corpora Wolffiana of higher animals, than with their true Kidneys 

 (CHAP- XV. 3). The vertebral column itself is often very imperfectly deve- 

 loped ; in a large proportion of the group, the skeleton is cartilaginous only ; 

 and in the lowest species, it does not even manifest a trace of division into 

 vertebrae. Living habitually in an element, which is nearly of the same speci- 

 h'c gravity with their own bodies, Fishes have no weight to support, and have 

 only to propel themselves through the water. Accordingly we find their 

 structure adapted rather for great freedom of motion, than for firmness and 

 solidity ; and as progressive motion is chiefly effected by the lateral action of 

 the spine, the vertebrae are so united, as to move very readily upon one ano- 

 ther. Instead of being articulated together by surfaces nearly flat, as in 

 Mammalia, or by ball-and-socket joints, as in Serpents, they have both their 

 surfaces concave : and these glide over a bag of fluid (the representative of the 

 intervertebral substance in the higher animals), which is interposed between 

 each pair. The tail is flattened vertically ; so as, by its lateral stroke, to pro- 

 pel the Fish through the water. By this character, true Fishes are distin- 

 guished from those aquatic Mammalia, which are adapted to inhabit their 

 element, and which commonly receive the same designation ; for the latter, 

 being air-breathing Animals, are obliged to come frequently to the surface to 

 respire ; and their tail is flattened horizontally, to enable them to do this with 

 facility. The lateral surface of the body of Fish is further extended above, by 

 the projection of the dorsal fin, which is supported on the spinous processes of 

 ihe vertebrae ; and below, by the abdominal fin, which also is placed on the 

 median line ; these will, of course, increase the power of the lateral stroke of 

 the body, and can only be moved with the spine. The pectoral and ventral 

 fins, on the other hand, the former of which answer to the superior extre- 

 mities, and the latter to the inferior extremities, of Man, serve, by their in- 

 dependent movements, rather as steering than as propelling organs ; and they 

 also assist in raising and depressing the animal through the water. The 

 scales with which the bodies of all Fishes are covered, are frequently of a 

 bony hardness, and sometimes form a firmly-jointed casing, in which the 

 trunk is completely inclosed ; this is especially the case, when the internal 



