312 
Bibliographical Notices. 
a series of long slender thread-like tentacula, — and thus it lives ap- 
parently for an indeterminate period, exercising all the functions of a 
perfect and adult animal even to the repeated production of young 
in all res})ects alike to the parent. So it lives until, from some un- 
known causes, a change comes over it, and it begins to unveil itself, 
and to exhibit one of the most wonderful revelations in animal trans- 
mutations. A pendulous column or roll is observed as if implanted 
on the disc of the hydra ; at first it is faintly indented by circles and 
is terminated by a circular row of tentacula ; the indenting circles 
become more deeply waved, the tentacula shorter until they are ob- 
literated ; and then each roll of the column is successively separated 
and liberated from the others until the whole embryonic column is 
dissolved, the individual rolls floating at freedom in the bosom of the 
waters, obviously the young of one of those large Medusae which 
swarm our seas in the months of the latter summer and autumn ! — 
Now this short sketch of the metamorphosis is not of any new dis- 
covery, for Sars had made us in some degree acquainted with it, but 
the account of it given by Sir John Dalyell excels all others in full- 
ness and completeness, and in its freedom from conjectural explica- 
tions. The metamorphosis itself is wonderfully curious, but what 
strikes us as the most unaccountable fact in the process is the un- 
certainty of the periods at which the change shall take place. The 
Hydra tuba shall remain for years a hydra propagating its kind, and 
we know of no data to fix the period when it shall begin the process 
of change into its mature and final state ; and, to add wond^ to 
wonder, having cast oflf several of these medusean embryos, a basis 
remains out of which another Hydra tuba shall arise, to go through 
the same hydra life and the same medusean metamorphoses as its 
predecessor. We suppose that these facts — for facts they are — will 
not support the opinions of Steenstrup on alternating generations, 
nor can even be reconciled with them. 
The way in which Sir John discovered that the Hydra tuba was 
the embryo of a Medusa was this : he took a large Medusa, of unde- 
termined species but beautifully figured on plate 15, and placing it 
in a vase of sea- water the spawn — “ a brownish matter like dust” — 
was shed from its ovarian fringes and settled at the bottom. This 
spawn consisted of “ an host of animated creatures in quick and va- 
ried motion,” partaking much of the nature of the planules of the Ser- 
tularians. The changes they rapidly underwent were noted and de- 
lineated ; and in eleven or twelve days after “ the planule had been 
discharged from the unwieldy Medusa, it was converted to a sta- 
tionary hydra.” (p. 105.) “ This new animal was provided with a 
complement of eight arms, yet so immature as to be of unequal 
dimensions. Different groups, under metamorphosis, showed the 
utmost irregularity in respect to evolution, to their shape and pro- 
portions : nor was it until thirteen days later, or three weeks after 
their birth, that any appeared with eight regular tentacula. Thus 
was a most perplexing problem solved — the Hydra tuba proved to 
have sprung of a Medusa.” (p. 105.) 
The progress of discovery went on. Sir John had “remarked 
