GENERAL BIOLOGY. 23 



conjugation" of higher forms, and is of great biological sig- 

 nificance. 



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 ia 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 BeU-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 oesophagus ; 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. 



MULTICEI.I.UI.AR ORGANISMS. 



The Fresh-Water Polyps {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 

 be fitly undertaken at this stage. The Polyps are easily obtain- 

 able from ponds in which they are found attached to various 

 kinds of weeds. To the naked eye, they resemble translucent 

 masses of jelly with a greenish or reddish tinge. They range 

 in size from one quarter to one half au inch ; are of an elongated 



