the sense of sight; they seem mere worms, furnished with fins and gills, and were so classed by Linnæus; but though now ascertained to be in reality fishes, they must be regarded as the lowest link in the scale--as connecting the class with the class _Vermes_, just as the superior cartilaginous fishes may be regarded as connecting it with the class _Reptilia_. [Footnote O: Some of the osseous fishes are also viviparous--the "viviparous blenny," for instance. The evidence from which the supposed affection of the higher fishes for their offspring has been inferred, is, I am afraid, of a somewhat equivocal character. The love of the sow for her litter hovers, at times, between that of the parent and that of the epicure; nor have we proof enough, in the present state of ichthyological knowledge, to conclude to which side the parental love of the fish inclines. The connubial affections of some of the higher families seem better established. Of a pair of gigantic rays (_Cephaloptera giorna_) taken in the Mediterranean, and described by Risso, the female was captured by some fishermen; and the male continued constantly about the boat, as if bewailing the fate of his companion, and was then found floating dead.--See Wilson's article Ichthyology, _Encyc. Brit._, seventh edition.] Between the osseous and the cartilaginous fishes there exist some very striking dissimilarities. The skull of the osseous fish is divided into a greater number of distinct bones, and possesses more movable parts, than the skulls of mammiferous animals: the skull of the cartilaginous fish, on the contrary, consists of but a single piece, without joint or suture. There is another marked distinction. The bony fish, if it approaches in form to that general type which we recognize amid all the varieties of the class as proper to fishes, and to which, in all their families, nature is continually inclining, will be found to have a tail branching out, as in the perch and herring, from the bone in which the vertebral column terminates; whereas the cartilaginous fish, if it also approach the general type, will be found to have a tail formed, as in the sturgeon and dog-fish, on both sides of the hinder portion of the spine, but developed much more largely on the under than on the upper side. In some instances, it is wanting on the upper side altogether. It may be as impossible to assign reasons for such relations as for those which exist between the digestive organs and the hoofs of the ruminant animals; but it is of importance that they should be noted.[P] It may be remarked, further, that the great bulk of fishes whose skeletons consist of cartilage have yet an ability of secreting the calcareous earth which composes bone, and that they are furnished with bony coverings, either partial or entire. Their bones lie outside. The thorn-back derives its name from the multitudinous hooks and spikes of bone that bristle over its body; the head, back, and operculum of