POLYGASTRICA. 51 



(71.) The character which distinguishes the class of microscopic 

 creatures which first offers itself for consideration, is derived from 

 the nature of the digestive apparatus with which the creatures com- 

 posing it are provided ;* this consists of a number of internal sacs 

 generally regarded as stomachs, which are easily distinguishable with 

 the microscope, and form a feature in their economy so peculiar, 

 that they are from this circumstance alone at once recognised as an 

 exceedingly natural and well-defined group, allied with each other in 

 the general details of their history, and exhibiting most astonishing 

 powers, not met with in other forms of being. In order to investigate 

 the facts which will be hereafter stated, connected with the history of 

 these animals, the young naturalist must be provided with a good 

 microscope, furnished with glasses capable of magnifying objects 

 from 200 to 1000 diameters, the last will be seldom needed ; but 

 a power of one-fourth of an inch focus will be indispensable. As 

 some practice and dexterity is requisite in prosecuting researches of 

 this description, a few hints relative to the best methods of procur- 

 ing and observing animalcules will not be improper in this place. 

 It would be needless to advert to the situations in which they are 

 to be found ; every stream and stagnant pool contains some forms 

 in countless numbers ; but, in order to obtain many uncommon 

 species, a little care is necessary. The lemna or duck-weed 

 should be skimmed from the surface of ponds which are exposed 

 to the rays of the sun, or the green film, which not unfrequently 

 covers stagnant waters ; and from these sources examples of most 

 tribes may readily be collected : or else recourse may be had to 

 infusions of various vegetable substances, of hay, chopped straw, 

 or the leaves of plants, which, if left in open glass vessels, and 

 fully exposed in the open air to the influence of the sun, will in a 

 few days swarm with polygastric animals, sometimes not to be pro- 

 cured by other means. 



A drop of water derived from any of these sources, if placed 

 upon a thin plate of glass, and covered with a film of talc, will rea- 

 dily enable the observer to examine the beings which inhabit it ; 

 or if it be deemed advisable to insulate the larger species, they 

 may be separated from the rest with a feather, and placed in small 

 tubes or flat troughs in filtered water, and their developement and 

 mode of increase watched from day to day. 



(72.) We shall now proceed to describe some of the most common 

 forms which the Polygastrica thus procured exhibit. In all water 



* Ehrenberg. 



E 2 



