same purpose, (Figs. 61-64.) Many Annelides, the leeches for example, have a sucker, which enables them to produce a vacuum, and thereby draw out blood from the perforations they make in other animals. Many microscopic animals are provided with hairs or cilia around the mouth, (Fig. 65,) which by their incessant motion produce currents that bring within reach the still more minute creatures or particles on which they feed. 225. Among the Vertebrata, the herbivora generally employ their lips or their tongue, or both together, for seizing the grass or leaves they feed upon. The carnivora use their jaws, teeth, and especially their claws, which are long, sharp even movable, and admirably adapted for the purpose. The woodpeckers have long, bony tongues, barbed at the tip, with which they draw out insects from deep holes and crevices in the bark of trees. Some reptiles also use their tongue to take their prey. Thus, the chameleon obtains flies at a distance of three or four inches, by darting out his tongue, the enlarged end of which is covered with a glutinous substance to which they adhere. The elephant, whose tusks and short neck prevent him from bringing his mouth to the ground, has the nose prolonged into a trunk, which he uses with great dexterity for bringing food and drink to his mouth. Doubtless the mastodon, once so abundant in this country, was furnished with a similar organ. Man and the monkeys employ the hand exclusively, for prehension. 226. Some animals drink by suction, like the ox, others by lapping, like the dog. Birds simply fil. the beak with. water, then, raising the head, allow it to run down into the crop. It is difficult to say how far aquatic animals require water with their food; it seems, however, impossible that they should swallow their prey without introducing at the same time some water into their stomach. Of many among the lowest animals, such as the Polyps it is well known that they frequently fill the whole cavity of their body with water, through the mouth, the tentacles, and pores upon the sides, and empty it at intervals through the same openings. And thus the aquatic mollusks introduce water into special cavities of the body, or between their tissues, through various openings, while others pump it into their blood vessels, through pores at the surface of their body. This is the case with most fishes. 226 a. Besides the more conspicuous organs above described, there are among the lower animals various microscopic apparatus for securing their prey. The lassos of polypi have been already mentioned incidentally, (223.) They are minute cells, each containing a thin thread coiled up in its cavity, which may be thrown out by inversion, and extend to a considerable length beyond the sac to which it is attached. Such lassos are grouped in clusters upon the tentacles, or scattered upon the sides of the Actinia and of most polypi. They occur also in similar clusters upon the tentacles and the disk of jelly-fishes. The nettling sensation produced by the contact of many of these animals is undoubtedly owing to the lasso cells. Upon most of the smaller animals, they act as a sudden, deadly poison. In Echinoderms, such as star-fishes, and sea-urchins, we find other microscopic organs in the form of clasps, placed upon a movable stalk. The clasps, which may open and shut alternately, are composed of serrated or hooked branches, generally three in number, closing concentrically upon each other. With these weapons, star-fishes not more than two inches in diameter may seize and retain shrimps of half that length, notwithstanding their efforts to disentangle themselves. CHAPTER SEVENTH. CF THE BLOOD AND CIRCULATION. 227. THE nutritive portions of the food are poured into the general mass of fluid which pervades every part of the body, out of which every tissue is originally constructed, and from time to time renewed. This fluid, in the general acceptation of the term, is called blood; but it differs greatly in its essential constitution in the different groups of the Animal Kingdom. In polypi and medusæ, it is merely chyme, (208;) in most mollusks and articulates it is chyle, (209;) but in vertebrates it is more highly organized, and constitutes what is properly called BLOOD. 228. The BLOOD, when examined by the microscope, is found to consist of a transparent fluid, the serum, consisting chiefly of albumen, fibrin, and water, in which float many rounded, somewhat compressed bodies, called blood disks. merous in birds than in mammals, and more abundant in the latter than in fishes. In man and other mammals they are very small and nearly circular, (Fig. 78;) they are somewhat larger, and of an oval form, in birds and fishes, (Figs. 79, 81;) and still larger in reptiles, (Fig. 80.) 229. The color of the blood in the vertebrates is bright red; but in some invertebrates, as the crabs and mollusks, the nutritive fluid is nearly or quite colorless; while in the worms and some echinoderms, it is variously colored yellow, orange, red, violet, lilac, and even green. 230. The presence of this fluid in every part of the body is one of the essential conditions of animal life. A perpetual current flows from the digestive organs towards the remotest parts of the surface; and such portions as are not required for nutriment and secretions return to the centre of circulation, mingled with fluids which need to be assimilated to the blood, and with particles of the body which are to be expelled, or, before returning to the heart, are distributed in the liver. The blood is kept in an incessant CIRCULATION for this purpose. 231. In the lowest animals, such as the polypi, the nutritive fluid is simply the product of digestion (chyme) mingled with water in the common cavity of the viscera, with which it comes in immediate contact, as well as with the whole interior of the body. In the jelly-fishes, which occupy a somewhat higher rank, a similar liquid is distributed by prolongations of the principal cavity to different parts of the body, (Fig. 31.) Currents are produced in these, partly by the general movements of the animal, and partly by means of the incessant vibrations of microscopic fringes, called vibratile cilia, which overspread the interior. In most of the mollusks and articulates, the blood (chyle) is also in immediate contact with the viscera, water being mixed with it in mollusks; the vessels, if there are any, not forming a complete circuit, but emptying into various cav ties which interrupt their course. 232. In animals of still higher organization, as the vertebrates, we find the vital fluid enclosed in an appropriate set of vessels, by which it is successively conveyed throughout the system to supply nutriment and secretions, and to the respiratory organs, where it absorbs oxygen, or, in other words, becomes oxygenated. 233. The vessels in which the blood circulates are of two kinds: 1. The arteries, of a firm, elastic structure, which may be distended or contracted, according to the volume of their contents, and which convey the blood from the centre towards the surface, distributing it to every point of the body. 2. The veins, of a thin, membranous structure, furnished within with valves, (Fig. 82, v,) which aid in sustaining the column of blood, only allowing it to flow from the periphery towards the centre. The arteries constantly subdivide into smaller and smaller branches; while the veins commence in minute twigs, and are gathered into branches and larger trunks, to unite finally into a few stems, near the centre of circulation. Fig. 82. 234. The extremities of the arteries and veins are con nected by a net-work of extremely delicate vessels, called capillary vessels, (Fig. 83.) They pervade every portion of the body, so that almost no point can be pricked without drawing blood. Their office is to distribute the nutritive fluid to the organic cells, where all the important processes of nutrition are performed, such as the alimentation and growth of all organs and tissues, the elaboration of bile, milk, saliva, and Fig. 83. |