HYDRA. 165 



larger polypi, which are generally met with as- 

 sembled in clusters; on which account it is termed 

 by Ellis the Actinia sociata, or cluster-animal flower. 

 It consists of a globular body, with a mouth sur- 

 rounded by one or two rows of tentacula ; termi- 

 nating below in a firm and fleshy tube, which ad- 

 heres strongly to the rocks at the bottom of the 

 sea ; so that it remains permanently fixed in the 

 same place. 



The genus Vorticella is formed by a small tribe 

 of animals, which, although they have been usually 

 included under the present order, differ from Polypi 

 in having no tentacula, but only cilia, surrounding 

 the margin of a bell-shaped body, which is mounted 

 upon a long and slender foot-stalk (Fig. 77).* Cur- 

 rents are, as usual, excited by the vibrations of tlie 

 cilia ; which in the simpler species, such as the 



Vorticella cyathina, here deline- 

 ated, are the efficient instruments 

 of progressive motion. When at- 

 tached by its foot, the vorticella 

 advances in search of food, by 

 the extension of the foot-stalk 

 into a straight line : but quickly 

 retreats from danger, by suddenly 

 throwing it into spiral folds. 

 Many of the species of vorticellae are so exceed- 

 ingly diminutive as to be imperceptible without 

 the aid of the microscope. They conduct us, there- 

 fore, by a natural gradation, to the next order we 

 have to notice, which is composed wholly of micro- 

 scopic animals. 



* They also differ from Polypi in having a distinct intestinal 

 canal, with numerous stomachs. The whole genus is now included 

 by Ehrenberg in his class of Polygastric Infusoria. 



