The eyes, though constructed on the same general principles as those of insects, yet present some particulars worthy of your notice. In the crabs and lobsters they consist of numerous facets, behind each of which is a conical or prismatic lens, the round extremity of which is fitted into a transparent conical pit, corresponding to a vitreous body, while the conical extremity of these lenses is received into a kind of cup, formed by the filaments of the optic nerve. Each of these filaments, together with its cup, is surrounded by pigment matter in a sheath-like manner. To see this structure would require anatomical skill; but you may here examine with a low power portions of the cornea, or glassy exterior, of the eye of a crab and of a lobster. In the former, you see that the facets into which the cornea is divided are hexagonal, like those of most insects, but in the latter they are square. But Crustacea have a far greater faculty of circumspection than insects have; for besides the extensive convexity and numerous facets of their eyes, these organs are placed at the extremity of shelly foot-stalks, which are themselves movable on hinges, capable of being projected at pleasure, of being moved in different directions, and of being packed snugly away, when not in active use, in certain grooves hollowed out expressly for them in the front margin of the shell. REPTILES --PETER MARK ROGET The order of Batrachia, or Amphibious Reptiles, constitutes the first step in the transition from aquatic to terrestrial vertebrata. It is more particularly the function of respiration that requires to be modified, in consequence of the change of element in which the animal is to reside; and as if it had been necessary, conformably to the laws of animal creation, that this change should not be abruptly made, we find that Batrachian reptiles, with which this series commences, are constructed, at first, on the model of fishes; breathing the atmospheric air contained in the water by means of gills, and moving through the fluid by the same instruments of progression as fishes, which, indeed, they exactly resemble in every part of their mechanical conformation. The tadpole, which is the young of the frog, is, at first, not distinguishable in any circumstance of its internal skeleton, or in the disposition of its vital organs, from the class of fishes. The head, indeed, is enlarged, but the body immediately tapers to form a lengthened tail, by the prolongation of the spinal column, which presents a numerous series of coccygeal vertebræ, furnished with a vertical expansion of membrane to serve as a caudal fin, and with appropriate muscles for executing all the motions required in swimming.