92 ANIMAL PHYSIOLOGY. 
other method of reproduction is known. A more or less globu- 
lar body encircled with a ring of cilia and of relatively small 
size may sometimes be seen attached to the usual form of Vorti- 
cella, 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 Amoeba 
itself belongs. It will be observed that the resemblance of this 
organism to Amceba 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 with- 
in one cell. Amoeba 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 cor- 
respondingly 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 neigh- 
borhood ; 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 cesophagus; while all the 
movements of the animal are rhythmical to a degree not exem- 
plified in such simple forms as Amoeba; 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 vari- 
ous directions by winds in the form of microscopic dust. 
MULTICELLULAR ORGANISMS. 
THE FRESH-WATER Po.yps (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 
