136 THE MECHANICAL FUNCTIONS. 



The Zoantlius (Fig. 58) belongs to a tribe of larger poly- 

 pi, which are generally met with assembled in clusters; on 

 which account it is termed by Ellis the Actinia sociata, or 

 clustering animal flower. It consists of a globular body, 

 leaving a mouth surrounded by one or two rows of tentacu- 

 ja; and connected below with a firm and fleshy tube, which 

 adheres 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 ani- 

 mals, which, although they have been usually included un- 

 der the present order, differ from polypi in having no tenta- 

 cula, but only cilia, surrounding the margin of a bell-shaped 

 body, which is mounted upon a long and slender foot-stalk 

 (Fig. 77.*) Currents are, as usual, excited by the vibra- 

 tions of the cilia; which in the simpler species, such as the 



Vorticella cyathina, here delineated, 

 are the efficient instruments of progres- 

 sive motion. When attached by its foot- 

 stalk, 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 exceedingly diminutive 

 as to be imperceptible without the aid of 

 the microscope. They conduct us, therefore, by a natural 

 gradation, to the next order we have to notice, and which 

 is composed wholly of microscopic animals. 



§ 4. Infusorna, 



The Infusory animalcules, or Infusoria, were so named 

 by Muller, a Danish naturalist, from the circumstance of 

 their swarming in all infusions of vegetable or animal sub- 

 stances that have been kept for a sufficient time. They are, 

 in general, far too minute to be perceptible to the naked eye: 

 it is to the microscope alone, therefore, that we owe our 



* They also dlffer'from polypi in having a distinct intestinal canal, with 

 numerous stomachs. 



