INTROD.] FOOD OF ANIMALS AND PLANTS. 23 



nutrient fluid takes place by a general surface. The Volvox globa- 

 tor has no inlet to its interior but through the pores in its walls. 

 A parasite of the human body, the Acephalocyst, also derives its 

 nutriment by imbibition through its walls. A familiar example is 

 the Acephalocystis endogena, or pill-box hydatid of Hunter. It con- 

 sists of a globular bag, closed at all points, containing a limpid fluid, 

 capable of growth, and of reproduction by the development of 

 gemmules from the inner surface of the sac. The Echinococcus is 

 also nourished by direct absorption into the walls of the globular 

 sac of which it consists. 



Some difference may be noticed as regards the nature of the food 

 in animals and plants. The former derive their nutriment entirely 

 from the organized world, unless, indeed, we suppose that the nitro- 

 gen absorbed in respiration contributes to their sustenance. Plants 

 appropriate inorganic elementary matters for food, as carbon, car- 

 bonic acid, ammonia, &c. " Inorganic matter," says Liebig, " affords 

 food to plants ; and they, on the other hand, yield the means of sub- 

 sistence to animals. The conditions necessary for animal and vege- 

 table nutrition are essentially different. An animal requires for its 

 development, and for the sustenance of its vital functions, a certain 

 class of substances which can only be generated by organic beings 

 possessed of life. Although many animals are entirely carnivorous, 

 yet their primary nutriment must be derived from plants ; for the 

 animals upon which they subsist receive their nourishment from 

 vegetable matter. But plants find new nutritive material only in 

 inorganic substances. Hence one great end of vegetable life is to 

 generate matter adapted for the nutrition of animals out of inor- 

 ganic substances which are not fitted for this purpose." 



The nutrient fluid, however formed, is distributed throughout the 

 textures of the plant, or animal, by vital or physical forces, or by 

 the junction of both ; and the function, by which this is effected, is 

 called Circulation. In plants, this function is very simple, and is 

 performed without the agency of a propelling organ ; but, in the 

 greatest number of animals, such an organ, a heart, is the main in- 

 strument in the distribution of the blood. In animals, then, there 

 is a true circulation ; the fluid setting out from, and returning to, 

 the same place. But, in plants, the fluid is found to circulate, or 

 rotate, within the interior of cells, as in Chara and Vallisneria, the 

 fluid of one cell not communicating with that of the adjacent ones, 

 or to pass up from the spongioles in an ascending current, and to 

 descend in another set of vessels. 



But in many simple animals, some entozoa, for example, and 



