THE sea teems with animals of gigantic bulk, but of inferior organization to those of the land or the air. The lowest animals of each class, and the lowest classes of this great kingdom of nature, are inhabitants of that dense element which encompasses three fourths of the surface of this planet. Relieved from muscular exertion by having their carcass suspended in a liquid medium, and stimulated incessantly on the surface by so dense an element, they remain nearer to their primitive condition.

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... here all animals are alike surrounded with a fluid,-their vital movements being directed to the elaboration of the simpler elements of structure,-the march of their internal development is arrested. This is not more manifest in the permanent tadpole condition of the fishes, than in the fish-like forms of the marine mammalia, which permanently retain the legless embryo state of our species, when the body consists, as in them, almost alone of the trunk and its caudal prolongation. Their most essential and most important organs, however, are constructed on a plan different from that of the fishes, or any other vertebrated forms we have hitherto considered, especially in all that relates to the production of the species; they are viviparous and mammiferous, like man himself, and form a part of the last and highest class of the animal kingdom. The massive and durable forms of mam. rnalia are supported by a strong and solid frame-work, the bones of which have a composition and texture between the dense and light air-tubes of birds, and the soft and spongy bones of reptiles. The solid walls which bound the periphery of the bones of mammalia, are thicker and stronger considerably than in birds. The cavities of the long bones are smaller, and more subdivided by internal cancelli, than those of birds, but they are larger than any cavities found in the cancellated bones of reptiles. In quadrupeds, those cavities are filled with marrow, which gives a certain degree of toughness and elasticity to the bones ; in birds, they are filled with air, to lighten the skeleton. The texture of the bones in quadrupeds is more coarse than in that of birds, and the fibrous structure is more obvious. There is a greater proportion of animal matter in them than in the bones of birds; they are, thus, tougher, less brittle, and by their greater thickness they are stronger. The skeleton of quadrupeds is constructed nearly upon the same general plan throughout all the orders of this class, in the essential parts or central portion of the body. The vertebral column, that series of hollow segments which forms the central pillar of the skeleton of vertebrated animals, is that part the variations of which are most characteristic of the different classes of these animals. We find, therefore, that there are characters of the bodies of the vertebræ, which we can trace through the class of birds, others which we can trace through the reptiles, other characters which distinguish the vertebrse of fishes. So with regard to the vertebral column of mammiferous animals. But as you pass anteriorly or posteriorly from the middle of the vertebral column in quadrupeds, you find modifications more and more ex-