72 THE ESSEX NATURALIST.
and since marsupia are used it is possible that the production
period is individually prolonged. Curiously the largest planulae
and strongest Hydra Tubae with Aurelia came from the smallest
Medusae.
Cyanea is the unknown quantity in the case. The ova are
developed in long thin and very delicate strings containing great
numbers, but they are very seldom ripe—in the Thames. In
this species, unless quite fully developed, the planulae simply
encyst in a minute tabloid form and never progress further:
and since in the congested waters of the Thames Estuary it is
normal for every Cyanea to sweep up with the folds, or curtains,
associated with its mouth parts, the planulae of other species of
Medusae in its immediate neighbourhood, it is quite common
to find the whole of the Hydra tubae, obtained from a Cyanea,
to belong to another species; its own planulae having all
encysted and become abortive.
Planulae were found to be free swimming from two to four
days in the summer, up to ten days in the autumn.
The next stage—or Hydra Tuba—is, it seems to me, the
fundamental stage, since it would seem to be possible for it to
maintain the species by this means alone, as a Hydra tuba can
produce numerous buds, send out shoots, or runners, and in
Aurelia push out cysts, a sort of egg5 (as with the freshwater
Hydra), which can develop into a Hydra tuba in fourteen days.
In the Hydra tuba stage the creatures have advanced in develop-
ment, for whilst in the planule it would seem they develop by
external absorption of nutrient fluids, in the Hydra tuba they
are furnished with an internal digestive process with gastric-
pouches, generally four in number, but in Cyanea varying up to
six or more. Like anemones, they are provided with a disc
supplied with a hypostome—or raised mouth—and surrounded
with a single marginal row of tentacles, usually sixteen in number,
but varying in the species and with age. This character, by
itself, is valueless from the point of view of specific identification.
Generally the transparency of the disc in the Hydra Tuba is not
very different in individuals, but in Cyanea capillata it is often
so transparent that one would hardly suppose that it existed,
and, unfortunately, Aurelia (in my third type) has also an
extremely transparent disc which is very misleading to anyone
5 It is not suggested that these are of sexual origin.