JELLYFISH. 73
not on the watch for it. This renders the character valueless
for purposes of specific identification. Cyanea Lamarckii has a
very transparent short stalk, and once again the Hydra tuba
of the third type possesses a very transparent stalk which, how-
ever, is usually very much longer relatively than that of Cyanea.
I fear I must diverge here to point out that the size of
medusae supplies no criterion for judging the size of the inter-
mediate form called Hydra tuba, the height of which when
attached to rock or weed can only be measured in a very few
millimetres (2 to 7 mm.), so that a lens of some sort is absolutely
essential for serious work on these creatures. It is safe to say
that relatively very few people have seen a living Hydra Tuba,
and when I commenced to collect in 1923 the total number in
the Natural History Museum at South Kensington was one—
under a lens—in the Great Hall. Yet the numbers of planulae
emitted annually must surely amount to billions! Most Hydra
Tubae possess a stalk, apparent or real, and all—on occasion—are
able to produce a very mobile process resembling a tentacle,
from a point about one-third the way up from the base; this
is used sometimes as an emergency holdfast but nearly always
for budding, the bud appearing as a small lump at any spot
in its length. In Miss Renton's Hydra Tuba this process is
used for every purpose needed by the creature; in the fourth
type of Aurelia, though possessed as a thin hair, it apparently
is never used at all on any occasion.
In Aurelia and Cyanea the apparent stalks are really exten-
sions of the body cavity, an economy of Nature, the expanded
portion above supplying, normally, all the area of surface
required for oxygenation, digestion and reproduction. At the
base of each polyp, even in Miss Renton's, there exist cells
which deposit fluid chitin, which, in turn, until hardened with
time, makes a sucker-like disc by which the Hydra Tubae are
enabled to attach themselves to various substances; attaching
better to some than to others. Stones, sand, shells, plants,
Polyzoans, glass, slate, crocks, all are accepted, but stones, chalk,
Polyzoans, Hydroids and algae would seem to be the natural
bases offered in nature.6 When placed under the microscope
the disc is seen to be a small cone and pressure ruptures
6 T. Tai Chuin (a Chinese) has done very nice work at Roscoff Biol. Station on the cells
depositing the chitin, and his results have been published in French.