and definitely limited body, within which is a complicated mass of viscera, while in the plant the physiological equivalents of these, the nutritive and respiratory organs, are distributed in areas of considerable extent over the diffused and indefinite outer surface of the body. In the animal the absorbent surfaces of these organs are internal; in the plant, external. In nearly all the higher animals there is a mouth opening, through which solid as well as liquid food may pass into the digestive cavity, which is furnished with specialized glandular appendages, such as the salivary glands, liver, etc. Within the digestive tract the food is elaborated and prepared for digestion and digested, and the indigestible refuse is discharged from the body through a definite anal opening. The nitrogenous products of decomposition are excreted from the body, usually in solution, by definite urinary organs. There is a muscular pulsating heart, by which the nutritive fluid or blood is propelled through blood-vessels with definite walls, and respiration is effected almost entirely by definite limited organs which are usually internal. The animal also has internal reproductive organs, as well as a nervous system and organs of sensation.
In the plants those organs which exist at all are present in a much simpler form. The roots absorb nutritive matter through their surfaces, usually as a fluid, and the surfaces of the leaves are the respiratory organs, absorbing and giving oil gases. The complicated system of internal organs, so characteristic of the animal, is entirely wanting in the plant, and the internal substance of the latter is made up of a comparatively homogeneous parenchyma of cells and tubes, through which the fluids circulate. The reproductive elements are not formed in limited local internal glands, but externally, and there are no nerves or sense-organs.
This distinction is diagnostic but not perfectly characteristic; that is, we may safely classify as an animal any organism in which we find a definite, sharply-limited body, and complicated internal viscera, such as a digestive tract, respiratory organs, blood-vessels, internal reproductive organs, and a nervous system and sense-organs; while we may, with almost equal safety, refer to the vegetable kingdom an organism in which the nervous, sensory, and circulating organs are wanting, and the processes of absorption and respiration take place through the outer surface. This distinction is therefore diagnostic, for it enables us to determine, with considerable certainty, to which of the two groups a given organism is to be referred; but it is not characteristic, and cannot be made the basis of an absolute definition, for it gradually disappears as we study the lower animals and plants. As a matter of fact, each one of the peculiarities given above as distinctive of animals will be found lacking in organisms the animal nature of which is undoubted, and many animals will be found to want all of them. Even among the vertebrates the organs of respiration are greatly simplified in the lower forms. In the adult frog the skin aids the lungs in aërating
