DIFFERENCES BETWEEN ANIMALS AND PLANTS. 33 



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 consider- 

 able 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 append- 

 ages, 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 defi- 

 nite 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 respi- 

 ration 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 sur- 

 faces, usually as a fluid, and the surfaces of the leaves are the respira- 

 tory 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 reproduc- 

 tive 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 un- 

 doubted, 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 aerating 



VOL. XIV. — 3 



