78
THE ESSEX NATURALIST.
Tubae seen by me are very much alike, but at the end of two or
three months if the conditions have suited them they have
diverged quite a lot. Unfortunately Aurelia, the most active
and virile of all species seen by me, tends to resemble the other
forms in one or the other of its types, and one has to seek for
characteristics other than shape alone, though Chrysaora is very
stable and generally identifiable with certainty apart from its
physiological differences. It is well to regard as Aurelia all
Hydra Tubae of unknown origin whose disc much exceeds the
column in diameter, Cyanea tending rather to be straight and
sometimes incurved; but one cannot rely upon this character-
istic as invariable, and consideration of the style of budding,
reaction to light, colouration, presence and use of the lateral
process, transparency or otherwise of disc, number of gastric
pouches, etc., is necessary in reinforcing any conclusion arrived at
before the final test of strobilation is reached.
With Chrysaora the "sunshine test" is final and conclusive
in the Hydra Tuba stage.
Knowing nothing about my subject when I started, I was
most exercised in my mind as to what would prove to be the
exciting cause of strobilation. I considered light and darkness
(shutting up Hydra Tubae in a perfectly dark cupboard for 21
days until they came out brown as a Barcelona nut); heat and
cold, leaving them out in 12° of frost Fahr.; food of various
sorts; position of polyp, pendant, detached or upright; stagna-
tion, or oxygenation, within my powers to give; no stone was
left unturned. Ultimately it appeared that it depended on
suitable food, a certain low critical temperature, varying with
species, and oxygenation at the critical period. This critical
low temperature must be attained and the seawater must be
"living," and not "dead," It is rather difficult to find words
to express the matter clearly; but there is water in which some
life cannot become manifest, yet it is not actually bad. It
is a degree of staleness, a question of P.H.
Strobilation lasts from January to February with Cyanea,
from December to March with Aurelia, and from February to
May (and probably later) with Chrysaora. All Hydra Tubae
pass through that most extraordinary metamorphosis known as
strobilation (from Strobila, the Artichoke). In this phase,
which takes from 14 to 21 days according to the lowness of the