24 
wide as possible, and struck out again in the same manner. It 
was easy to recognise them as the meduse of Hydra Tuba. The 
polypes from which they have been thrown off, have evidently been 
brought into the tanks on the shells of the oysters, which we have 
found so useful in rendering the sea-water clear and transparent. 
Of this I shall have more to say on a future occasion. But. these 
beautiful little fellows have a wonderful history attached to them. 
The transformation they undergo are almost beyond belief. 
People are so familiar with the fact that the brightly coloured 
butterfly or moth has once been a crawling caterpillar, and after- 
wards a dull chrysalis incapable of locomotion, that they have 
almost ceased to regard thesé developments as remarkable, and 
look on them as a matter of course. But the changes of form and 
habit in many marine creatures which take place during their 
varied life, are equally surprising and more complicated ; and, 
owing to the difficulty of observing them, are not so well known 
to the general public. For instance, the little acorn barnacle 
(Balanus balanoides), whose conical house is seen affixed to almost 
every stone and shell taken from the sea, was once an active little 
thing, capable of free motion through the water. The home- 
staying oyster in its childhood paddled itself about by means of its 
ciliz wherever it listed, in a manner which you have before heard 
described ; the jelly-like ascidian was once a minute tadpole ; and 
I might cite dozens of similar instances of marine animals enjoying 
activity in youth, and subsiding afterwards into sedentary maturity. 
But the transformations of the medus@ are quite as wonderful. 
The common jelly-fish, or ‘‘ moon-fish”’ (Medusa chrysaora), which 
must be familiar to everyone who has had a sea-side holiday, has 
been observed (by Sir J. Dalyell) to produce minute ova, from 
which proceed embryos called planula, little crawling organisms, 
which by-and-by attach themselves to the rocks, and become meta- 
morphosed into stationary polypes. In this condition, as Hydra 
tuba, they were brought into the tanks of the Brighton Aquarium, 
parasitically affixed to oyster-shells. Hydra tuba has the appea- 
rance of a little cone with tentacles, holding by its pointed end to 
the surface to which it is attached. At a certain period its outline 
appears corrugated, and a closer examination shows these corruga- 
tions to consist of so many little saucers, piled up one within the 
other. The topmost saucer detaches itself and swims away ; and 
the rest, one by one, follow its example until the cone of Hydra 
tuba is dispersed, and the little saucers are seen to be so many 
meduse, such as are now abundant in the Brighton tanks, and 
some of which are on this table. At this stage our real knowledge 
of their history ceases. It is believed that they continue to grow 
until they become the large jelly-fishes which are so common in 
our seas, but I am not aware that any observations have been 
made of their intemediater development. 
