Page:A Text-book of Animal Physiology.djvu/52

 method of reproduction is known. A more or less globular body encircled with a ring of cilia and of relatively small size may sometimes be seen attached to the usual form of Vorticella, with which it finally becomes blended into one mass. This seems to foreshadow the "sexual conjugation" of higher forms, and is of great biological significance.

Vorticella may pass into an encysted and quiescent stage for an indefinite period and again become active. The history of the Bell-animalcule is substantially that of a vast variety of one-celled organisms known as Infusoria, to which Amœba itself belongs. It will be observed that the resemblance of this organism to Amoeba is very great; it is, however, introduced here to illustrate an advance in differentiation of structure; and to show how, with the latter, there is usually a physiological advance also, since there is additional functional progress or division of labor; but still the whole of the work is done within one cell. Amœba and Vorticella are both factories in which all of the work is done in one room, but in the latter case the machinery is more complex than in the former; there are correspondingly more processes, and each is performed with greater perfection. Thus, food in the case of the Bell-animalcule is swept into the gullet by the currents set up by the multitudes of vibrating arms around this opening and its immediate neighborhood; the contractile vesicles play a more prominent part; and the waste of undigested food is ejected at a more definite portion of the body, the floor of the œsophagus; while all the movements of the animal are rhythmical to a degree not exemplified in such simple forms as Amœba; not to mention its various resources for multiplication and, therefore, for its perpetuation and permanence as a species. It, too, like all the unicellular organisms we have been considering, is susceptible of very wide distribution, being capable of retaining vitality in the dried state, so that these infusoria may be carried in various directions by winds in the form of microscopic dust.

MULTICELLULAR ORGANISMS. (Hydra viridis; Hydra fusca). The comparison of an animal so simple in structure, though made up of many cells, as the Polyp, with the more complex organizations with which we shall have especially to deal, may