552 POLYGASTRIC ANIMALCULES; VORTlCELLINjE. 



powerful ; and the Animalcules show by their movements a 

 great susceptibility to external impressions. Thus, the common 

 Vorticella (Fig. 626, a), which clusters round the stalks of duck- 

 weed, is usually attached by a long peduncle, which, when the 

 animal is seeking for its prey by the vibration of its cilia, seems 

 fully extended, and almost put on the stretch. But if the stage of 

 the miscroscope be smartly tapped, the Animalcule is seen to 

 contract its peduncle suddenly ; and it afterwards slowly extends 

 it when free from alarm. In the Stentor (Trumpet Vorticella), 

 which is one of the largest of the Polygastrica, the body is itself 

 prolonged, and attached at its basis. In general these Animal- 

 cules have the power of separating themselves from their attach- 

 ment, retracting the foot into the body, so that no appearance 

 of a peduncle is seen, and then swimming freely in search of a new 

 point to which to fix themselves. Some of them produce buds 

 from their sides, like the Hydra ; and others multiply by sub- 

 division. Both these processes take place, if the temperature be 

 genial, and the animals be well supplied with food, with great 

 rapidity. By the older observers, those which were seen 

 during their change, and whose form was thus very different 

 from the one usually observed, as also those that were seen in their 

 unattached condition, were regarded as distinct species. Errors 

 of this kind have been by no means uncommon in Natural 

 History, and will always be committed whilst observers are 

 content with the cursory inspection of external forms, and do 

 not make the changes, which these forms may undergo in the 

 progress of life, an object of special study. In some genera 

 belonging to this group, we observe the stalks of a number of 

 individuals proceeding from one common stem ; and sometimes 

 this ramifies and subdivides, so as to exhibit a completely 

 arborescent form, analogous to that of the Polypifera. The 

 circular arrangement of the cilia around the mouth, and the 

 mode in which these organs are employed in obtaining food, has 

 occasioned the Vorticellce and the Wheel- Animalcules to be con- 

 founded together. 



1 132. The almost universal diffusion of this class of Animal- 

 cules is not the least astonishing part of their history. There is 



