54 BIOLOGY: GENERAL AND MEDICAL 



mines all the bacteria, flagellates, and ciliates to the 

 edges of the glass or to the air bubbles caught in the 

 liquid. 



Hertwig also points out that if a plasmodium of Aetha- 

 lium septicum be enclosed in a cylindrical vessel contain- 

 ing boiled water, the vessel closed with a perforated cork 

 and inverted over a dish of fresh water, the plasmodium 

 soon escapes from the boiled water to the aerated water 

 through the opening in the cork. 



If fishes be kept in an aquarium without plants or 

 other means of aerating the water, they will be found to 

 behave in a peculiar manner as the oxygen becomes 

 exhausted. They first rise to the surface near which 

 they remain. As the available oxygen diminishes, they 

 swim along the surface with the mouth open until a 

 globule of air is secured and forced to the back part of the 

 mouth in order that it may be brought into contact with 

 the gills. By this means asphyxia may be postponed 

 for some time. 



When the higher animals are excluded from oxygen a 

 complicated series of nervous and muscular manifesta- 

 tions, collectively known as asphyxia, occurs. They are 

 chiefly characterized by involuntary muscular efforts, 

 brought about through excitation of the automatic res- 

 piratory centres, the purpose of which is to relieve the 

 organism of the accumulated CO 2 and to secure fresh O. 

 As the condition progresses the innervation being more 

 and more profoundly disturbed, the movements become 

 more and more violent until the animal falls convulsed 

 and exhausted. 



HELIOTROPISM OR RESPONSE TO PHOTIC STIMULATION. 



With but few exceptions living organisms are sensitive 

 to light and react according to conditions not all of which 

 are understood. 



It is found by experiment that the different rays of the 

 solar spectrum have varying effects upon living organ- 



