464 GENERAL HISTORY OF THE INEUSOEIA. 



this season until some future period, since observations prove their power of 

 assuming a torpid condition when existing circumstances are unfavourable 

 to the full exercise of life. It has been noticed by Ehrenberg, of some Eota- 

 toria li\ing, so to speak, in confinement, or in a limited quantity of water 

 under examination, that when the weather was warm and nourishment 

 abundant, life was prolonged to 18 or 20 days and more ; and Mr. Gosse also 

 speaks of a MeUcerta which hved in confinement for 14 days. 



The conditions of life, or the causes affecting the vitahty of Rotifera 

 favourably or unfavourably, are in some respects very remarkable, as an ap- 

 peal to their habitats alone would abundantly illustrate. 



It is during the height of summer that the Rotifera are multiplied most 

 abundantly ; but when the cold frosty nights of autumn supervene, their 

 numbers undergo a rapid reduction. However, often during the most beauti- 

 ful parts of the year, as Perty remarks, a sudden decrease ^vill occur. 



" Two kinds of disease," writes Ehrenberg, '' destroy the Hijdatina and 

 most of the Rotatoria : 1, the formation of vesicles, which give rise to the 

 appearance of small rings all over the creature ; and, 2, the formation of 

 granules which so penetrate the internal organs that these seem composed of 

 them, and have a shagreen appearance." The first condition has been noted 

 by Weisse, who regards the apparent vesicles as parasitic organisms. 



The Rotatoria also suffer from the overgrowth upon their surface of Algae 

 and of parasitic animals, Protozoa and the like, and are at length destroyed 

 thereby. Eoul or decomposing water is incompatible with their existence, as 

 are some chemical mixtures, whilst to others they seem indifferent. Thus 

 Hydatince have been fed A^dth rhubarb and indigo in powder without sensible 

 effect, and neither calomel nor corrosive sublimate Idlls them ; at least, they 

 live for some time after these substances have been mixed with the water. 

 Strychnia causes instant death. 



The deprival or the want of renewal of aii' in water inhabited by Rotifera 

 causes their destruction, for example, when collected in abottle for examination, 

 the cork being allowed to remain too long. In like manner the exclusion of air 

 by a pellicle of oil on the surface of the water, or the withdrawal of air by 

 means of an au' pump, speedily destroys the Rotatorial inhabitants. Ehren- 

 berg affirms that they exist much longer in an atmosphere of nitrogen than in 

 one of carbonic acid or of hydi'ogen, and that the vapour of sulphur speedily 

 puts an end to their existence. 



Still a very imperfect renewal of air seems, at least in some instances, to 

 suffice — as in the case of the Rotifer vulgaris and R. parasita, which have 

 been seen within the spheres of Volvox and in the cells of aquatic plants 

 (the Vaucheria clauata.) Perty likewise mentions the Notommata Werneclcii 

 as inhabiting the Vaucheria ccesjpitosa ; and Albertia vermicularis is parasitic 

 within the intestine of earth-worms and slugs. In all these instances Hfe is 

 compatible with a very slight renewal of atmospheric air, or, iu fact, is sup- 

 ported amid the gases generated within these organic beings and mixed with 

 their fluids. 



The evaporation of the water from around Rotifera, as when under exami- 

 nation by the microscope, is a frequent cause of their destruction, by the 

 breaking up of their soft parts. But there is a happy provision against such 

 evil consequences ; for, so soon as the animal experiences the deficiency of 

 water around it, it withdi'aws its tender wheel apparatus, and limits its ex- 

 posed parts as much as possible, by retracting its pseudopodium and contract- 

 ing itself into a ball-like form, so that only the denser integument is exposed 

 to the injurious influences, and the evaporation of water from the contained 

 organs reduced to its minimum. 



