IXFUFOEIA. 59 



3. Classification. — Of the numerous me- 

 thods of arranging the Infusoria which, at differ- 

 ent times, have been proposed, those of Ehren- 

 berg, Dujardin, and Claparede appear to be most 

 worthy of attention. All these classifications must, 

 however, be regarded as premature, since we know 

 so little of the life-history of these animals that 

 it is by no means improbable that many appa- 

 rently distinct species are nothing more than 

 transitional conditions of more adult forms. It is 

 now many years since it was satisfactorily demon- 

 strated by Cohn, that at least e'lgld of Ehrenberg's 

 genera were merely so many different stages in the 

 development of one of the lower Algae. Hence, 

 in the follo\\dng account of the Infusoria, it will be 

 desirable to confine ourselves to the description of 

 those more important characteristic featm-es which 

 have been made the subject of renewed and care- 

 ful investigation, directing attention, as we pro- 

 ceed, to those members of the group, in which 

 such characteristics may most readily be observed. 



4. Siizp. — The Infusoria vary considerably in 

 size, the greater number being invisible without 

 the assistance of the microscope. Thus the ave- 

 rage length of the body of Vorticelkif exclusive of 

 the stalk, may be estimated at '003 of an inch. 

 Stentor {fig. 13, a), which is, perhaps, the largest 

 of all Infusoria, attains a length of -04 of an inch, 

 whilst others are so minute as to present the ap- 

 pearance of mere moving points under the higher 

 powers of the most improved instruments. But, 

 since the true nature of these last can be judged 

 of only by analogy, it seems probable that they 

 ought rather to be regarded as vegetable monads, 



