If we consider only the organization and nature of animals, without regard to their size, utility, the greater or less knowledge we have of them, and other accessory circumstances, we shall find there are four principal forms, four general plans, if it may be so expressed, on which all animals seem to have been modeled, and whose ulterior divisions, whatever be the titles with which naturalists have decorated them, are merely slight modifications, founded on the development or addition of certain parts, which produce no essential change in the plan itself. In the first of these forms, which is that of man and of the animals most nearly resembling him, the brain and principal trunk of the nervous system are inclosed in a bony envelope, formed by the cranium and vertebræ; to the sides of this intermedial column are attached the ribs and bones of the limbs, which form the framework of the body; the muscles generally cover the bones, whose motions they occasion, while the viscera are contained within the head and trunk. Animals of this form we shall denominate Animalia Vertebrata. They have, all, red blood, a muscular heart, a mouth furnished with two jaws situated either above or before each other, distinct organs of sight, hearing, smell, and taste placed in the cavities of the face, never more than four limbs, the sexes always separated, and a very similar distribution of the medullary masses and the principal branches of the nervous system. By a closer examination of each of the parts of this great series of animals, we always discover some analogy, even in species the most remote from each other; and may trace the gradations of one same plan from man to the last of the fishes. In the second form there is no skeleton; the muscles are merely attached to the skin, which constitutes a soft contractile envelope, in which, in many species, are formed stony plates, called shells, whose position and production are analogous to those of the mucous body. The nervous system is contained within this general envelope along with the viscera, and is composed of several scattered masses connected by nervous filaments; the chief of these masses is placed on the æsophagus and is called the brain. Of the four senses, the organs of two only are observable, those of taste and sight, the latter of which are even frequently wanting. One single family alone presents organs of hearing. There is always, however, a complete system of circulation, and particular organs for respiration. Those of digestion and secretion are nearly as complex as in the vertebrata. We will distinguish the animals of this second form by the appellation of Animalia Mollusca. Although, as respects the external configuration of the parts, the general plan of their organization is not as uniform as that of the vertebrata, there is always an equal degree of resemblance between them in the structure and the functions.