INFUSORIAL ANIMALCULES. 19 



is such, that they appear to revolve together like a wheel upon its axle, and so 

 complete is the illusion that the name of wheel-animalcules, or Rotatoria, is given 

 to those which possess this peculiarity. 



Besides these organs, stiff hairs or bristles are found upon animalcules, which, 

 unlike the cilia, are devoid of rotation, but serve as supports to the body, and 

 also aid these living atoms in climbing. Animalcules are also found with hook- 

 like projections extending from the under side of the body, which are capable of 

 motion to some extent, but do not possess the peculiar movement of the cilia. 

 Many Infusoria are also endowed with another kind of member, that more com- 

 pletely subserves the purpose of motion, and which they have the power of pro- 

 truding or withdrawing at pleasure, as the snail extends and retracts its horns. 

 These organs are soft, and by some species can be thrust out from every part of 

 the body; while in others that are partially covered by a shell, they are confined 

 to the uncovered portions. 



The power of extension possessed by Infusoria over these organs is much 

 greater, in proportion to their size, than in the case of snails and animals of a 

 similar nature. 



In those Infusoria that are gifted with the highest organization, as the wheel- 

 bearing animalcules, there appears to be a member resembling a claw, by means 

 of which they attach themselves firmly to any object within their grasp. The 

 claw is appended to an extended portion of the body, resembling a foot. 



CLASSIFICATION. Dr. Ehrenberg, to whom we are more indebted than to any 

 other observer, for our knowledge of Infusoria, divides this living world into two 

 great classes, distinguished from each other by their structure : viz., the Poly- 

 gastrica* or many-stomached animalcule, and the Rotatoriaf or wheel-animalcule. 



POLYGASTRICA. If an animalcule of this class is viewed by the microscope, a 

 number of round spots within its body will be readily detected, which are often 

 quite large compared with the size of the living atom. These spots are so many 

 stomachs, connected together by a single tube, and forming the digestive appa- 

 ratus of the creature. If the water around the animalcule is clear, the stomachs 

 will appear more transparent than the rest of the body ; but if it is tinted with 

 sap-green or carmine (which substances are usually employed) they will be seen 

 more distinctly ; for the animalcule readily imbibes the colored fluid, and the 

 stomachs from their transparency then appear of the same hue as the liquid ; 

 while the tint of the more solid portions of the body remains unchanged. The 

 number of stomachs varies in different species from four to upwards of two hundred. 



In the annexed cut a highly magnified view of a bell-shaped animalcule is 

 presented, in which the stomachs and coronets of cilia are distinctly exhibited. 



None of this class of infusoria are more than the twelfth of an inch long, and 

 the smallest species, when full grown, do not exceed in extent the thirty-six thous 



* From the Greek polus, many, and gaster, a stomach f From the Latin rota, a wheel. 



