INTRODUCTION TO ZOOLOGY. 



(1,620 feet), and, consequently, subjected to a pressure equal 

 to 50 atmospheres.* Nor are they bounded even by these 

 localities, for they have been discovered in the cells of plants, 

 and in other situations where, but a few years ago, their pre- 

 sence would not have been suspected. 



As they are so widely diffused, and must, in such variet^ 

 of circumstances, subsist on veiy different kinds of food, it may 

 naturally be expected that they must present very considerable 

 diversity of size, form, and structure. These differences 

 furnish means by which species can be distinguished from 

 each other; the agreement of several species in some one 

 common character enables the naturalist to combine them into 

 one genus ; and, by a repetition of the same process, to unite 

 several genera into one larger group, on which some common 

 and characteristic name is bestowed. In this way, the whole 

 of the Infusoria may be arranged in two great divisions. The 

 distinguishing characteristic of the first of these is the presence 

 in the body of the creature of a number of sacs, or stomachs, in 

 which the food is received ; and from this peculiarity the order 

 is called Polygastrica, or "many-stomached" (Fig. 2). In the 

 second order, instead of this pecu- 

 liarity, there is another not less re- 

 markable. About the head there are 

 rounded lobes, which, when looked 

 at under the microscope, seem like 

 wheels in rapid motion; and hence 

 the creatures in which this was ob- 

 served were called "wheel-animal- 

 cules," and the order itself JRotifera, 

 or "wheel-bearing." The parts do 

 not in reality move like wheels, but 

 the movements of the delicate hair- 

 like organs with which they are 

 fringed make them seem to do so. 



The use of scientific terms has 

 something in it very repulsive to the 

 young naturalist. But this often 

 arises from the terms being used without any precise idea of 

 their meaning being conveyed to the mind of the learner. 

 When any term is thoroughly understood, there is, an end of the 



2. LEUCOPIIUYS. 



* About 750 Ibs. on each square inch of surface. 



