PROTOZOA AND THE UTRICLES OF UTRICULARIA. 247 



of another bladder; the entire larva was instantly drawn in, 

 although the bladder was hardly longer than the larva. A 

 living insect larva of the same length was placed at the entrance 

 of another bladder; when it touched the lower edge of the valve 

 it was also entirely and instantly drawn in. Part of a slender 

 branch of a utricularia plant was presented to another bladder; 

 this was drawn in until one end touched the posterior end of the 

 bladder and the other end projected from the entrance. No 

 more of this branch was taken in during the succeeding 24 hours. 



It is evident from these experiments that the animals taken into 

 the bladders do not try to force an entrance, but are sucked in 

 when, by their movements, they stimulate the plant in such a 

 way as to cause the valve to open and the walls of the bladder to 

 expand. The exact mechanism of this process is still to be 

 worked out. 



Other species of Utricularia probably capture their prey in a 

 similar manner. One of these Utricularia intermedia Hayne, 

 which bears bladders on separate leafless branches that lie on 

 the bottom was examined at Salisbury Cove, Maine; its bladders 

 were compressed, but expanded when stimulated with a needle 

 just as did those of U. vulgaris v. americana. 



Various investigators have attempted to determine how utricu- 

 laria bladders capture their prey. Darwin carried on experi- 

 ments somewhat similar to those described above but failed. 

 That he very nearly succeeded is evident from the following 

 account of his experiments. 



"As I felt much difficulty in understanding how such minute and weak animals, 

 as are often captured, could force their way into the bladders, I tried many experi- 

 ments to ascertain how this was effected. The free margin of the valve bends so 

 easily that no resistance is felt when a needle or thin bristle is inserted. A thin 

 human hair, fixed to a handle, and cut off so as to project barely J of an inch, 

 entered with some difficulty; a longer piece yielded instead of entering. On three 

 occasions minute particles of blue glass (so as to be easily distinguished) were 

 placed on valves whilst under water; and on trying gently to move them with a 

 needle, they disappeared so suddenly that, not seeing what had happened, I thought 

 that I had flirted them off; but on examining the bladders, they were found safely 

 enclosed. The same thing occurred to my son, who placed little cubes of green 

 box-wood (about 1/60 of an inch, .423 mm.) on some valves; and thrice in the 

 act of placing them on, or whilst gently moving them to another spot, the valve 

 suddenly opened and they were engulfed. To ascertain whether the valves were 

 endowed with irritability, the surfaces of several were scratched with a needle or 



