26 NATURAL HISTORY 



one or two species) : they are colourless and transparent 

 as the clearest crystal, yet can no internal organization be 

 seen^ excepting that connected with their digestive func- 

 tion, which consists of two or more globular cavities or 

 sacs, probably communicating with each other by a tu- 

 bular membrane, as in the larger polygastric animalcules, 

 but which in this genus is too minute to be discerned ; 

 indeed, the stomachs or sacs themselves are only to be 

 observed when the animalcule is fed with particles of 

 colouring matter : the food on which they usually exist 

 being as pellucid as themselves, the cavities are invisible. 

 They increase by a spontaneous division of the parent 

 into two or more parts, and those parts, or young, again 

 divide when they have attained their full size. 



As subjects of observation they are principally interest- 

 ing from their minuteness, being as they are the very 

 limit of man's acquaintance with animated nature. Their 

 diameters vary from I -24000th of an inch to 1- 1200th, 

 and consequently require a very high magnifying power 

 to discern. They are numerous, and generally found 

 congregating at the surface, and around the decomposed 

 matter of infusions, either of vegetable or animal sub- 

 stances. Muller enumerates ten species, but Ehrenberg 

 has included fifteen. 



l. MoNAS termo. The End Monad. — This animalcule, 

 as its name imports, is a mere point, even when viewed 

 under considerable magnifying powers ; indeed it is so 

 very minute, that its existence cannot be discovered in 



