74
THE ESSEX NATURALIST.
it in the flattening, showing its real design very clearly.
Anyone seeing Hydra Tubae would deride the idea of their
possessing colour, as the minute polyps in Nature simply
show a "frosty transparency" like ground glass. When
overfed, however, they mostly appear white, though when
Aurelia is photographed they appear in the shadings appro-
priate to yellow, pink or green; and, in fact, these colours
are actually present. Aurelia Type 1 has yellow, Type 2 pink
(or mauve), Type 3 yellow, Type 4 glaucous (or drab), Chrysaora
glaucous or drab, Cyanea both forms iridescent bronze, capillata
in special circumstances, scarlet; and Miss Renton's Hydra
Tuba, when heavily fed, exhibits a deep beetroot-crimson.
Under natural conditions none of these colours is at all likely
to be observed. The hypostome is very much the same with
all, except Miss Renton's Hydra Tuba, in which example it is
most inconspicuous. The labial extremity of a well developed
Cyanea Hydra Tuba showed itself to be more mobile than that
of Aurelia of corresponding size. In Aurelia and Cyanea when
well fed the stalk disappears in the deepening of the column, but
Chrysaora retains its stalk, as likewise does Miss Renton's Hydra
Tuba. In both of these cases, however, it would seem that the
stalk is really the lateral process specifically adapted for holding
purposes, and it is curious to note that in both species the polyps
are asymmetrical. There are, however, two great differences
between the two forms. Miss Renton's Hydra Tuba buds con-
tinuously and most profusely and Chrysaora hardly at all:
Rentoni can attach and detach itself instantly, whereas Chrysaora
evinces great difficulty in providing sufficient or suitable chitin
for reattachment. Rentoni can also swim with ease, both by
cilia and by movements of the stalk, and Chrysaora swims feebly
when young. Both detest light, though their reactions are very
different. Rentoni contracts to a blob and sinks; Chrysaora
contracts its tentacles to mere knobs, and withdraws its
hypostome, contracting its disc, on being exposed to sun-
shine, until it is totally obliterated, the polyps becoming
open cups. This is a most interesting physiological character-
istic and definitely of specific value. Both are deep water
forms ; Chrysaora a bottom form and Rentoni a planktonic
form; the latter, however, is not able to stand very low tempera-
tures. It is probable that Chrysaora utilizes plants, kelp, etc., and