Chap. XVII. MANNER OF CAPTURING PREY. 407 



long splinter, of whicli one end rested obliquely on 

 the valve, and after a few hours it was found fixed, half 

 within the bladder and half projecting out, with the 

 edge of the valve fitting closely all round, except at 

 one angle, where a small open space was left. It was 

 so firmly fixed, like the above mentioned larvae, that 

 the bladder was torn from the branch and shaken, and 

 yet the splinter did not fall out. My son also placed 

 little cubes (about -^V of an inch, '391 mm.) of green 

 box- wood, which were just heavy enough to sink in 

 water, on three valves. These were examined after 

 19 hrs. 30 m., and were still lying on the valves ; but 

 after 22 hrs. 30 m. one was found enclosed. I may 

 here mention that I found in a bladder on a naturally 

 growing plant a grain of sand, and in another bladder 

 three grains ; these must have fallen by some accident 

 on the valves, and then entered like the particles 

 of glass. 



The slow bending of the valve from the weight of 

 particles of glass and even of box-wood, though largely 

 supported by the water, is, I suppose, analogous to the 

 slow bending of colloid substances. For instance, 

 particles of glass were placed on various points of 

 narrow strips of moistened gelatine, and these yielded 

 and became bent with extreme slowness. It is much 

 more difficult to understand how gently moving a 

 particle from one part of a valve to another causes it 

 suddenly to open. To ascertain whether the valves 

 were endowed with irritability, the surfaces of several 

 were scratched with a needle or brushed with a fine 

 camel-hair brush, so as to imitate the crawling move- 

 ment of small crustaceans, but the valve did not 

 open. Some bladders, before being brushed, were left 

 for a time in water at temperatures between 80^ and 

 130° F. (26°-6— 54°-4 Cent.), as, judging from a wide- 



