GENERAL REMARKS — CONCLUSION. 87 



Though the influence of light is favourahle to infu- 

 sorial life, it does not appear to be absolutely necessary, 

 for many kinds of animalcules abound in the waters of 

 deep mines, which are always in impenetrable dark- 

 ness. 



The ordinary duration of life in the Infusoria varies 

 from a few hours to several days, or even weeks : Ro- 

 tifera have been traced to the twenty-third day of their 

 existence. The death of these animals is generally 

 sudden; but, in some of the larger species, convulsive 

 struggles attend their dissolution. Shortly after death, 

 the soft parts rapidly decompose, and all traces of their 

 beautiful structures disappear: the species which are 

 furnished with earthy cases or shells, alone leave durable 

 vestiges of their existence *. • 



The exceedingly small proportion of solid matter 

 that enters into the composition of these animals is re- 

 markable: from aggregated myriads and myriads, a 

 few atoms of dust alone remain! If it be supposed 

 that the bodies of the Infusoria are too minute to afford 

 any conclusive evidence on this point, it may be re- ' 

 marked that the same phenomenon appears in certain 



* The oceans that deposited those ancient sedimentary strata, in which 

 no traces of organic remains have been observed, (and are therefore called 

 by some geologists azoic), may, consequently, have been as densely peo- 

 pled with gelatinous organisms as the existing seas. See Appendix. 



