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



the prominent characteristic of the Articulata ; both, however, being rendered 

 subordinate to the great purpose to be attained in their fabric, the develop- 

 ment of an organ, through which intelligence peculiarly manifests itself. For 

 the operations of this, a degree of general perfection is required, which is not 

 met with elsewhere. The higher Vertebrata have a power of constantly 

 keeping the temperature of the body up to a point, which it can only attain 

 occasionally, and under peculiar circumstances, in the Articulata, and which 

 it never reaches in the Mollusca. This involves an energetic performance of 

 the functions of respiration and circulation ; and these again require consider- 

 able activity of digestion. All the Vertebrata have red blood, which is pro- 

 pelled through the system by a distinct muscular heart ; and the number of 

 corpuscles, which any given amount of the fluid contains, bears a nearly con- 

 stant proportion to the ordinary temperature of the animal. They are further 

 distinguished from Articulata by a character which seems of little importance, 

 but which is very constant in each group. Whilst the mouth of the 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. 



General characters of Fishes. 



37. 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, from the circumstance 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 subse- 

 quently dependent upon its parent for support. In many respects, the organ- 

 ization of Fishes is not much advanced beyond that of the higher Mol- 

 lusca. Their respiratory apparatus has the same character; and the organs 

 by which the blood is depurated of its superfluous azote, rather correspond 

 with the temporary Corpora Wolffiana of higher animals, than with their 

 true kidneys ( 669, 670). The vertebral column itself is often very imper- 

 fectly developed; in a large proportion of the group, the skeleton is cartila- 

 ginous only; and in the lowest species, it does not even manifest a distinct 

 division into vertebrae. Living habitually in an element which is nearly of 

 the same specific gravity with their own bodies, Fishes have no weight to sup- 

 port, and have only to propel themselves through the water. Accordingly we 

 find their structure adapted rather for great freedom of motion than for firm- 

 ness 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 another. 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 representa- 

 tive of the invertebral substance in the higher animals) which is interposed 

 between each pair. The tail is flattened vertically; so as, by its lateral 

 stroke, to propel the Fish through the water. By this character, true 

 Fishes are distinguished from those aquatic Mammalia, which are adapted to 



