THE WATER SPIDER. 



399 



Water Spider. 



detailed account. How the air is introduced into the cell is a 

 problem that was for some time unsolved. The reader is prob- 

 ably aware that the bubbles of air which are to be seen on sub- 

 aquatic plants are almost entirely composed of oxygen gas, which 

 is exuded from the plant, and which is so important an agent in 

 purifying the water. Some zoologists thought that the air which 

 is found in the cell of the "Water Spider was nothing but oxygen 

 that had been exuded from the plant upon which the nest was 

 fixed, and that it had been intercepted in its passage to the sur- 

 face. In order to set the question at rest, Mr. Bell, the well- 

 known naturalist, instituted a series of experiments upon the 

 Spider, and communicated the results to the Linnsean Society. 

 The experiments were made in 1856, and Mr. Bell's remarks are 

 as follows : 



" No. 1. Placed in an upright cylindrical vessel of water, in 



