416 REPORT—1863. _ 
mantias, and he saw some of them fix themselves to the sides of the vessel 
and develope a lobed disc. From this disc arose a stem, which developed 
from its summit a polypite closely resembling the Campanularia raridentata 
of Alder. 
In the Aquorea vitrina of Gosse, Wright * also observed free ciliated 
planule to escape from the generative bodies, and, after fixing themselves to 
the sides of the vessel, become developed into a hydroid, with polypite, 
hydrotheca, and periderm, bearing, as he informs us, a close resemblance 
to the Laomedea acuminata of Alder. 
The class of observations here enumerated enable us to complete the circle 
of hydroid-development ; for they prove that the ova of the free medusa 
undergo, like those of the fixed gonophore, a continuous development, by 
which they become transformed into polypoid trophosomes ; these trophosomes, 
as has been proved by direct observation in the case of Stawridium, and 
as we may unquestionably assume in other cases, giving origin, by buds, 
to meduse identical with those from whose ova the trophosome was directly 
developed. 
Relation between Sexual and Non-seaual Reproduction in the Hydroida.— 
The phenomena now described under the head of sexual and non-sexual 
reproduction present intimate and important relations with one another— 
relations which find their expression in the remarkable law, originally hinted 
at by Chamisso when he made his memorable discovery of the true genetic 
relations between the solitary Salpe and the associated chain-like colonies 
of these animals, but first distinctly enunciated by Steenstrup, under the name 
of the “Alternation of Generations ”—a law which, though in its original mode 
of statement it must undergo some modification, has nevertheless received, in 
all essential points, abundant confirmation, and will explain, in a way which 
it alone can do, a host of phenomena which would otherwise have appeared 
isolated and exceptional y. 
The law in question manifests itself among the Hyprorpa in the fact that 
between every two sexual zooids one or more non-sexual zooids intervene, 
the immediate result of the development of the ovum being in such cases 
always a non-sexual form. 
To take for example one of the simplest cases, we find that from the 
ovum of Clava multicornis there is developed directly a ciliated planula, 
passing by continuous metamorphosis into the non-sexual polypite. From 
this polypite there is then produced by gemmation a sexual zooid—the 
sporosac, which gives origin to ova or spermatozoa, destined to repeat the 
series, which thus consists of non-sexual polypite and sexual sporosac, and 
so on indefinitely. 
The “period” here indefinitely repeated in the life of the species consists 
accordingly of two terms—polypite—sporosac. 
Again, in Hydractinia echinata we have a case not quite so simple ; for 
here, while the ovum becomes developed as before into a polypite, this 
polypite, instead of directly producing gonophore-buds, sends off from its 
basal extension or ccenosare a peculiar bud, which, though still non-sexual, 
differs from the polypite, and has its alimentary functions suppressed. It 
constitutes the gonoblastidium, and is destined to give origin by budding to 
sporosacs. 
* Micr. Journ. vol. iii. pl. 4. figs. 1-6. ; 
t+ The true significance of the phenomena on which the law of ‘alternation of genera- 
tions” has been founded was for the first time clearly pointed out by Dr. Carpenter. See 
Brit. and For. Med. Chir. Rev. vol. i. Jan. 1848, p. 183, &e. 
