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 movement 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- [page 408] spread
analogy, this would have rendered them more sensitive to irritation, or
would by itself have excited movement; but no effect was produced. We
may, therefore, conclude that animals enter merely by forcing their way
through the slit-like orifice; their heads serving as a wedge. But I am
surprised that such small and weak creatures as are often captured (for
instance, the nauplius of a crustacean, and a tardigrade) should be
strong enough to act in this manner, seeing that it was difficult to
push in one end of a bit of a hair 1/4 of an inch in length.
Nevertheless, it is certain that weak and small creatures do enter, and
Mrs. Treat, of New Jersey, has been more successful than any other
observer, and has often witnessed in the case of Utricularia
clandestina the whole process.* She saw a tardigrade slowly walking
round a bladder, as if reconnoitring; at last it crawled into the
depression where the valve lies, and then easily entered. She also
witnessed the entrapment of various minute crustaceans. Cypris was
quite wary, but nevertheless was often caught. Coming to the entrance
of a bladder, it would sometimes pause a moment, and then dash away; at
other times it would come close up, and even venture part of the way
into the entrance and back out as if afraid. Another, more heedless,
would open the door and walk in; but it was no sooner in than it
manifested alarm, drew in its feet and antenn , and closed its shell.
Larv , apparently of gnats, when feeding near the entrance, are pretty
certain to run their heads into the net, whence there is no retreat. A
large larva is sometimes three or four hours in being swallowed, the
process bringing to
* New York Tribune, reprinted in the Gard. Chron. 1875, p. 303.
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