the hard shell, whilst in polyps they perforate the walls of the general cavity of the body, which they constantly fili with water. of Vertebrates. In those Radiates which are provided with distinct respiratory organs, such as the Echinoderms, we find still another typical structure, their gills forming bunches of fringes around the mouth, or rows of minute vesicles along the radiating segments of the body. 11* CHAPTER NINTH. OF THE SECRETIONS. 261. W LE, by the process of digestion, a homogeneous fluid is prepared from the food, and supplies new material to the blood, another process is also going on, by which the blood is analyzed, as it were; some of its constituents being selected and so combined as to form products for useful purposes, while other portions of it which have become useless or injurious to the system are taken up by different organs, and expelled in different forms. This process is termed SECRETION. 262. The organs by which these operations are performed are much varied, consisting either of flat surfaces or membranes, of minute simple sacs, or of delicate elongated tubes, all lined with minute cells, called epithelium cells, which latter are the real agents in the process. Every surface of the body is covered by them, and they either discharge their products directly upon the surface, as on the mucous membrane, or they unite in clusters and empty into a common duct, and discharge by a single orifice, as is the case with some of the intestinal glands, and of those from which the perspiration issues upon the skin, (Fig. 94.) 263. In the igher animals, where separate organs for special purposes are multiplied, numerous sacs and tubes are assembled into compact masses, called glands. Some of these are of large size, such as the salivary glands, the kidneys, and he liver. In these, clusters of sacs open into a common canal, and this canal unites with similar ones forming larger trunks, such as we find in the salivary glands, (Fig. 93,) and finally they all discharge by a single duct. Fig. 93. 264. By the organs of secretion, two somewhat different purposes are effected, namely, fluids of a peculiar character are selected from the blood, for important uses, such as the saliva, tears, milk, &c., some of which differ but little in their composition from that of the blood itself, and might be retained in the blood with impunity; or, the fluids selected are such as are positively injurious, and cannot remain in the blood without soon destroying life. These atter are usually termed EXCRETIONS. 265. As the weight of the body, except during its period of active growth, remains nearly uniform, it follows that it must daily lose as much as it receives; in other words, the excretions must equal in amount the food and drink taken, with the exception of the small proportion discharged by the alimentary canal. Some of the most important of these outlets will be now indicated. 266. We have already seen (37) that all animal tissues admit of being traversed by liquids and gases. This mutual transmission of fluids from one side of a membrane to the other is termed endosmosis and exosmosis, or imbibition and transudation, and is a mechanical, rather than a vital, phenomenon, inasmuch as it takes place in dead as well as in living tissues. The blood vessels, especially the capillaries. share this property. Hence portions of the circulating fluids escape through the walls of the vessels and pass off at the surface. This superficial loss is termed exhalation. It is most active where the bloodvessels most abound, and accord ingly is very copious from the air-tubes of the lungs and from the skin. The loss in this way is very considerable; and it has been estimated that, under certain circumstances, the body loses, by exhalation, five eighths of the whole weight of the substances received into it. This removal is some- 267. The skin, or outer envelop of the body, is otherwise largely concerned in the losses of the body. Its layers are constantly renewed by the tissues beneath, and the outer dead layers are thrown off. times gradual and continual, as in man. mollusks, it comes off in the form of slime, which is, in fact, composed of cells detached from the surface of the skin. Sometimes the loss is periodical, when it is termed moulting. Thus, the mammals cast their hair, and the deer their horns, the birds their feathers, the serpents their skins, the crabs their test, the caterpillars their outer envelop, with all the hairs growing from it. 268. The skin presents such a variety of structure in the different groups of animals as to furnish excellent distinctive characters of species, genera, and even families, as will hereafter be shown. In the vertebrates we may recognize several distinct layers, of unequal thickness, as may be seen in figure 94, which represents a magnified section of the human skin, traversed by the sudoriferous canals. The lower and thickest layer, (a,) is the cutis, or true skin, and is the part which is tanned into leather. Its surface presents numerous papillæ, in which the nerves of general sensation terminate; they also contain a fine network of bloodvessels, The usually termed the vascular layer. superficial layer (c) is the epidermis, or cuticle. The cells of which it is composed are distinct at its inner portion, but become dried and flattened as they are pushed outwards. It is supplied with neither vessels nor nerves, and, consequently, is insensible. Between these two layers, and more especially connected with the cuticle, is the rete mucosum, (b,) a very thin layer of cells, some of which contain the pigment which gives the complexion to the different races of men and animals. The scales Fig. 94. of reptiles, the nails and claws of mammals, and the solid coverings of the Crustacea, are merely modifications of the epidermis. On the other hand, the feathers of birds and the scales of fishes arise from the vascular layer. 269. Of all the Excretions, if we except that from the Lungs, the bile seems to be the most extensive and important; and hence a liver, or some analogous organ, by which bile is secreted, is found in animals of every department; while some, or all, of the other glands are wanting in the lower classes of animals. In Vertebrates, the liver is the largest of all the organs of the body. In mollusks, it is no less preponderant. In the gasteropods, like the snail, it envelopes the intestine in its convolutions, (Fig. 52 ;) and in the acephala, like the clam and oyster, it generally surrounds the stomach. In insects it is found in the shape of long tubes, variously contorted and interlaced, (Fig. 51.) In the Radiata, this organ is largely developed, especially among the echinoderms. In the star-fishes it extends into |