ORGANS OF DIGESTION. 309 



food is conveyed into them, and from its not being ac- 

 cumulated or retained in any other part of the digestive 

 apparatus. Nearly two hundred stomachs have been count- 

 ed in a paramcecium and in an aurelia, filled with food at the 

 same time, and there may have been many more, unseen from 

 their empty and collapsed state. These digestive sacs are 

 contracted filiform and almost invisible when empty, but they 

 are susceptible of remarkable dilatation, and are sometimes 

 seen distended with water, or smaller animalcules, or portions 

 of confervre swallowed as food ; and the forms of these minuter 

 animalcules can often be detected in the half-digested masses 

 expelled from their posterior opening. Viewed through the 

 microscope, the polygastric animalcules present very dif- 

 ferent appearances, according to the quantity and the kind of 

 food contained in these digestive sacs, and from deceptions 

 of this kind twelve different species of animalcules, belong- 

 ing to six supposed distinct genera, have been formed of the 

 single species vorticella convallaria. Although no muscular 

 apparatus is perceptible in the transparent bodies of the 

 polygastrica, distinct maxillary or dental organs are seen in 

 many species belonging to very dissimilar genera. They 

 consist of numerous long straight, stiff, elastic spines, dispos- 

 ed parallel to each other, and arranged so as to form an oral 

 cylindrical proboscis, capable of being extended, widened, or 

 contracted, to seize and compress the soft prey. No gland- 

 ular organs to assist in digestion have been observed in this 

 class of animals, notwithstanding their dental apparatus and 

 the multiplied cavities of their alimentary canal. They are 

 often observed to swallow animalcules nearly as large as 

 themselves, and which could not be lodged in any of the 

 digestive sacs ; these appear to remain in the distended ali- 

 mentary canal, and they render them, for a time, inactive* 

 One polygastric animalcule often contains many hundreds of 

 smaller prey within its body, and, notwithstanding the 

 almost invisible minuteness of the animals of this class, and 

 the great simplicity of their structure,, they appear to be at 

 once the most numerous, the most active, the most prolific, 

 and the most voracious of all living beings. In some of the 

 minutest forms of monads we are often unable to perceive 

 any internal cavity ; in others, from one to a very variable 

 number of cavities are rendered visible by coloured food an 



