III. The department of MOLLUSKS is divided into three classes, namely: 1. Those which have arms about the mouth, like the cuttle-fish, (Cephalopods,) fig. 47. 2. Those which creep on a flattened disk or foot, like snails, (Gasteropods,) fig. 88. 3. Those which have no distinct head, and are inclosed in a bivalve shell, like the clams, (Acephals.) + The CEPHALOPODS may be divided into a. The cuttle-fishes, properly so called, (Teuthideans,) b. Those having a shell, divided by sinuous partitions The GASTEROPODS contain four orders: a. The land snails which breathe air, (Pulmonates.) c. Those which have wing-like appendages about the d. A still lower form allied to the Polyps by their general appearance, (Rhizopods or Foraminifera.) The class of ACEPHALS contains three orders: a. Those having shells of two valves, (bivalves,) like the clam and oyster, (Lamellibranchiates.) . Those having two unequal valves, and furnished with peculiar arms, (Brachiopods.) c. Mollusks living in chains or clusters, like the Salpa, fig. 135; or upon plant-like stems, like Flustra, (Bryozoa.) IV. The department of RADIATES is divided into three classes: 1. Sea-urchins, bearing spines upon the surface, (Echin• oderms,) figs. 12, 26. 2. Jelly-fishes, (Acalephs,) fig. 31. 3. Polyps, fixed like plants, and with a series of flexible arms around the mouth, figs. 48, 77, 143. The ECHINODERMS are divided into four orders: ' a. Sea-slugs, like biche-le-mar, (Holothurians.) b. Sea-urchins, (Echini,) fig. 26. c. Free star-fishes, (Asteridæ,) fig. 17. d. Star-fishes mostly attached by a stem, (Crinoids,) figs. 150, 151. The ACALEPHS include the following orders: a. Those furnished with vibrating hairs, by which they move, (Ctenophora.) b. The Medusa, or common jelly-fishes, (Discophora,) figs. 31, 142. c. Those provided with aerial vesicles, (Siphonophora.) The class of POLYPS includes two orders. a. The so-called fresh-water polyps, and similar marine forms, with lobed tentacles, (Hydroïds,) fig. 143. b. Common polyps, like the sea-anemone and coral. polyp, (Actinoids,) fig. 48. In addition to these, there are numberless kinds of micro scopic animalcules, commonly united under the nan.e of infusory animals, (Infusoria,) from their being found specially abundant in water infused with vegetable matter. These minute beings do not, however, constitute a natural group in the Animal Kingdom. Indeed, a great many that were formerly supposed to be animals are now found to be vegetables. Others are ascertained to be crustaceans, mollusks, worms of microscopic size, or the earliest stages of development of larger species. In general, however, they are exceedingly minute, and exhibit the simplest forms of animal life, and are now grouped together, under the title of Protozoa. But, as they are still very imperfectly understood, notwithstanding the beautiful researches already published on this subject, and as many of them are likely to be finally distributed among vegetables, and the legitimate classes in the Animal Kingdom to which they belong, we have not assigned any special place for them. PHYSIOLOGICAL ZOOLOGY. CHAPTER FIRST. THE SPHERE AND FUNDAMENTAL PRINCIPLES OF ZOÖLOGY. 1. ZOOLOGY is that department of Natural History wh.cl relates to animals. 2. To enumerate and name the animals which are found on the globe, to describe their forms, and investigate their habits and modes of life, are the principal, but by no means the only objects of this science. Animals are worthy of our regard, not merely when considered as to the variety and elegance of their forms, or their adaptation to the supply of our wants; but the Animal Kingdom, as a whole, has a still higher signification. It is the exhibition of the divine thought, as carried out in one department of that grand whole which we call Nature; and considered as such, it teaches us most important lessons. 3. Man, in virtue of his twofold constitution, the spiritual and the material, is qualified to comprehend Nature. Being made in the spiritual image of God, he is competent to rise to the conception of His plan and purpose in the works of Creation. Having also a material body, like that of other animals, he is also in a condition to understand the mechanism of organs, and to appreciate the necessities of matter, as well as the influence which it exerts over the intellectual element throughout the domain of Nature. 4. The spirit and preparation we bring to the study of Nature, is a matter of no little consequence. When we would study with profit a work of literature, we first endeavor to make ourselves acquainted with the genius of the author; and in order to know what end he had in view, we must have regard to his previous labors, and to the circumstances under which the work was executed. Without this, although we may perhaps enjoy its perfection as a whole, and admire the beauty of its details, yet the spirit which pervades it will escape us, and many passages may even remain unintelligible. 5. So, in the study of Nature, we may be astonished at the infinite variety of her products; we may even study some portion of her works with enthusiasm, and nevertheless remain strangers to the spirit of the whole, ignorant of the plan on which it is based, and fail to acquire a proper conception of the varied affinities which combine beings together, so as to make of them that vast picture in which each animal, each plant, each group, each class, has its place, and from which nothing could be removed without destroying the proper meaning of the whole. 6. Besides the beings which inhabit the earth at the pres ent time, this picture also embraces the extinct races which are now known to us by their fossil remains only. And these are of the greatest importance, since they furnish us with the means of ascertaining the changes and modifications which the Animal Kingdom has undergone in the suc |