containing dead and captured animals usually include bubbles of air,
but these cannot have been generated solely by the process of decay, as
I have often seen air in young, clean, and empty bladders; and some old
bladders with much decaying matter had no bubbles.
The real use of the bladders is to capture small aquatic animals, and
this they do on a large scale. In the first lot of plants, which I
received from the New Forest early in July, a large proportion of the
fully [page 405] grown bladders contained prey; in a second lot,
received in the beginning of August, most of the bladders were empty,
but plants had been selected which had grown in unusually pure water.
In the first lot, my son examined seventeen bladders, including prey of
some kind, and eight of these contained entomostracan crustaceans,
three larv of insects, one being still alive, and six remnants of
animals so much decayed that their nature could not be distinguished. I
picked out five bladders which seemed very full, and found in them
four, five, eight, and ten crustaceans, and in the fifth a single much
elongated larva. In five other bladders, selected from containing
remains, but not appearing very full, there were one, two, four, two,
and five crustaceans. A plant of Utricularia vulgaris, which had been
kept in almost pure water, was placed by Cohn one evening into water
swarming with crustaceans, and by the next morning most of the bladders
contained these animals entrapped and swimming round and round their
prisons. They remained alive for several days; but at last perished,
asphyxiated, as I suppose, by the oxygen in the water having been all
consumed. Freshwater worms were also found by Cohn in some bladders. In
all cases the bladders with decayed remains swarmed with living Algae
of many kinds, Infusoria, and other low organisms, which evidently
lived as intruders.
Animals enter the bladders by bending inwards the posterior free edge
of the valve, which from being highly elastic shuts again instantly. As
the edge is extremely thin, and fits closely against the edge of the
collar, both projecting into the bladder (see section, fig. 20), it
would evidently be very difficult for any animal to get out when once
imprisoned, and apparently they never do escape. To show how closely
the edge [page 406] fits, I may mention that my son found a Daphnia
which had inserted one of its antenn into the slit, and it was thus
held fast during a whole day. On three or four occasions I have seen
long narrow larv , both dead and alive, wedged between the corner of
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