ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 421 
their adult state, while the phenomena presented by the other instances— 
those in which gemmation has not been directly observed—render it highly 
probable that the young forms seen in such instances are due also to an act 
of budding, followed, as in those cases where budding has been proved, by 
metamorphic changes; but whether in any of these cases the bud is destined 
to repeat exactly the parent form, or is only one term in a more extended 
life-series, we have not yet evidence to enable us to determine; while in no 
instance is there any proof whatever of the direct development of the medusa 
from the egg without the intervention of a non-sexual trophosome. 
In two families of undoubted hydroid meduse some observations have 
also been made, showing that the medusa passes during the free period of its 
existence through a series of well-marked metamorphoses before arriving at 
the adult state, resembling in this respect the Ayinide whose metamorphoses 
have just been described. 
Two such cases have been recorded. The first is one mentioned by 
Gegenbaur*, and adduced by him as a case of direct development from the 
egg. He describes a minute animal which he discovered swimming free in 
the sea. It was in shape somewhat like an inverted flask, measuring about 
0-05 of a line in diameter, having the base of the neck surrounded by a circle of 
three or four short tentacles, with four minute lithecysts between them. The 
entire animal, even to the tips of the tentacles, was thickly clothed with fine 
cilia. 
He traced it through various successive stages, in which he observed, first, 
an increase in the length and number of the tentacles; then the formation of 
an internal cavity, and a widening of the body into a more disc-like form, 
accompanied by an elongation of the neck; then the circumference of the 
disc-like body became extended laterally, and gradually assumed an umbrella 
shape, while the base of the internal cavity at the same time extended itself 
into eight radiating tubes, which, at the circumference of the disc, became 
united by acircularcanal. It had thus assumed a complete medusa-form 7 of 
a line in diameter; and we now find it with numerous rigid tentacula, eight 
radiating canals, lithocysts, and a two-lobed mouth ; while the cilia have at the 
same time disappeared from the general surface of the body, remaining only on 
that of the tentacles, which appear to retain them through life. Gegenbaur 
refers his medusa to a new genus and species under the name of T’rachynema 
ciliata, which he regards as the type of a distinct family not far removed 
from the Eucopide ; but though he had met with specimens measuring 1 line 
in the diameter of the umbrella, he never witnessed in them the occurrence 
of sexual elements. 
Here again, though an undoubted metamorphosis from a free-swimming 
ciliated form is apparent, there is no evidence whatever to show that the 
earliest observed stage had proceeded directly from the egg—a conclusion to 
which Gegenbaur seems to have been chiefly led by the ciliated condition of 
this stage. We have, however, already seen that a ciliated surface affords no 
proof of direct development from the egg. 
The other case is one recorded by Fritz Miillert, who gives us an account 
of the metamorphosis of Lyriope Catharinensis, Fr. Miill., a medusa belonging 
to the family of the Geryonide. He obtained in the sea at Santa Catharina 
spherical transparent bodies from 0-2 to 0-3 mm. in diameter. In the interior 
of these was aspherical cavity, situated so excentrically that at one side it was 
separated from the external water only by a thin plate. He next observed 
_* Generationswechsel, p. 51. 
+ Wiegmann’s Archiv, 1859, p. 310. 
