546 INFLUENCE OF LIGHT AND HEAT ON ANIMALCULES. 



possibly be unfounded. There can be little doubt, from their 

 position and their similarity to the undoubted visual organs in 

 the class nearest allied to them, that the red spots on the neck 

 of the Rotifera ( 857) niay be fairly regarded as eyes. But 

 the same arguments by no means hold good in regard to the 

 Polygastrica. 



1122. Light has certainly, however, a considerable influence 

 on these Infusoria, both in regard to their first production, 

 and their subsequent movements. It has been found that they 

 will appear much more speedily in an infusion exposed to light, 

 than in one secluded from it. But there are some species which 

 can exist in almost total darkness ; being found in the recesses of 

 the deepest mines. And it is further observed, tha,t a very 

 powerful light is injurious to them, and that they seek to avoid 

 it ; this may be due, however, to the heat which accompanies it, 

 of the influence of which they are very susceptible. The greater 

 number are instantaneously killed by a moderate heat, which 

 acts equally upon both ova and adults ; but they can support it 

 better if the temperature be raised gradually. There are a few 

 species which can support a temperature of 120. On the other 

 hand, cold is very injurious to these Animalcules ; but it affects 

 the adult more than the germ. If the water in which they 

 exist be suddenly congealed, they are usually killed within two 

 hours ; but they are at first able to develope heat enough to 

 prevent the freezing of their own bodies, as appears from the 

 small quantity of water which remains fluid around them. If 

 the congelation be gradual, however, they retreat beneath the 

 surface, so as not to be included in the coating of ice ; and in the 

 winter, therefore, we meet with them just below the frozen 

 covering of the waters which they inhabit. Even if they are 

 entirely inclosed, and the greater part of them destroyed, it is 

 seldom that one or two individuals do not remain, the eggs 

 being preserved where the parents have perished ; and from these 

 a new population, as great as the previous one, will speedily, 

 originate ( 1123) at the return of the genial warmth which 

 calls them into active life. 



1123. The presence of Oxygen in the atmosphere, with 



