INFUSORIA 457 



The history of the Vorticella, as it has been elaborately 

 worked out by Dr. Stein, exhibits phenomena analogous 

 to those marvellous changes which we lately considered 

 under the appellation of the Alternation of Generations. 

 Large individuals withdraw their circle of cilia, close up 

 the mouth, and become globular, and then secrete from 

 their whole surface a gummy substance, which hardens 

 into a spherical transparent shell, enclosing the Vorticella 

 in its cavity, in the form of a simple vesicle. Within this 

 vesicle is seen the band-shaped nucleus, unchanged, and 

 what was the contractile bladder, which, however, no 

 longer contracts. 



By and by this torpid Vorticella enlarges itself ir- 

 regularly, pushing out its substance in tufts of threads, 

 and frequently protruding from one side a larger mass, 

 which becomes an adhering stalk. Thus it has become 

 an Acmeta, such as we now behold. 



From this condition two widely different results may 

 proceed. In the one case, the encysted Vorticella sepa- 

 rates itself from the walls of the Acineta, and contracts into 

 an oval body, furnished at one end with a circle of vibra- 

 tory cilia, by whose movements it rotates vigorously in its 

 prison, while the more obtuse end is perforated by a 

 mouth leading into an internal cavity. In the interior of 

 this active oval body there are seen the band-like nucleus, 

 and a cavity which has again begun to contract and to 

 expand at regular intervals. It is, in fact, in every re- 

 spect like a Vorticella vase, which has just freed itself 

 from its stalk. Presently, the perpetual ciliary action so 

 far thins away the walls of the Acmeta that they burst at 

 some point or other, and the little Vorticella breaks out 



of prison, and commences life afresh. The Acineta t mean- 



SCIENCE 20 



