Recent Researches oti ifie Iiifmoria. 291 



the water ; lastly, tliey were repeatedly seen after the young 

 had been excluded from them, and when nothing remained but 

 the outer covering or shell. 



3. The rate of increase is very rapid, but varies according 

 as the animals are sparingly or abundantly supplied with food* 

 Dr Ehrenberg put into some of the vessels in which the animals 

 were kept, a small quantity of the green matter which collects 

 in stagnant water, and which itself consists of a species of infu- 

 soria {Manas pulvisctdus), having satisfied himself that it con- 

 tained none of the hydatinae. This substance was readily eaten 

 by the hydatinae, and produced a marked increase in their fe- 

 cundity. In one glass, for example, the original animal had, in 

 the course of nine days, produced only one young one and one 

 egg ; but on adding some of the green matter, the number in- 

 creased, in about twenty-four hours, to nine animals, besides 

 one egg. From this and similar instances, he reckons that, on 

 the most moderate computation, a single individual may, in 

 twenty days, increase to a million, and in twenty-four days, very 

 nearly to the enormous number of seventeen millions ; a rate of 

 increase far exceeding any thing observed in the rest of the ani- 

 mal creation. 



The remarkable effect of food which was observed in these 

 cases, affords a striking illustration of the general law, that the 

 propagation and increase of the lower tribes of animals are 

 greatly influenced by external circumstances. Another remark- 

 able instance of the same truth is furnished by the history of 

 the fresh- water polypi, in which animals Trembley found the 

 rapidity of propagation to be greatly dependent on the supply 

 of food, and on the degree of temperature to which they were 

 exposed. Cold invariably diminished their fecundity, and heat, 

 within certain limits, as constantly increased it. It is to be 

 regretted, that Dr Ehrenberg did not direct his attention to the 

 ; .fifiect of temperatMre on the propagation of the infusoria, 

 rjii He next proceeded to make similar observations on the class 

 . « ^f Poly gastric infusoria, of which he selected Paramecium «w- 

 ' relia and Stylonychia Mytilus as examples. The vaults pb- 

 tained with these species are as follows : - 



1. Propagation took place by a transverse division of the pa- 

 rent animal into two. He did not meet with instances of longi- 



t2 



