M. C. Mereschkowsky on the Hydroida. 251 
cylinder with a single diplopolar axis, in this respect differing 
in no way from the first appearance of a tentacle upon a 
hydranth, or of a medusa. In fact it is impossible to distin- 
guish a tentacle, a medusa, and a hydranth at the first moment 
of their appearance (fig. 9) ; each of them 
is merely an Archhydra or a Protohydra, 
which, if we accept the biogenetie law, 
leads us to believe that they are all dif- 2 
ferent modifications of a single primitive 
organism, and that they are all homolo- (A A 
| c 2] 
| 
gous. 
At any rate, I believe that to regard 
a (stauraxonic) hydranth as a colony of a Young hydrotheca. 
{monaxonic) Archydre is to look at the ® te Bh eg 
affair as it is fundamentally. We Bt Sirah 
Let us further remember two interesting Hydroids, namely 
Ophiodes mirabilis* and Ophiodes parasiticus, Sars}, which, be- 
sides the tentaculiferous individuals (colonies according to me), 
have monaxonic individuals, without tentacles, and absolutely 
presenting no difference from the tentacles of certain Hydroids. 
And this case proves further that the tentacle (an individual), 
which cannot serve the colony either by procuring or by 
digesting food, only remains useful to it by serving to defend 
it, a function which induced the capitate form of the tentacles. 
Tn fact, the monaxonic individuals have no mouth, and there- 
fore do not aid in nutrition; and, at the same time, they are 
often placed so far from the colony-individuals that they can- 
not serve for seizing food. ‘The function of defence, therefore, 
alone remains for them; and we find that they have acquired 
the capitate form, which we have seen to be appropriated to 
defence. 
Fig. 9. 

II. Remarks on the Reproduction of Obelia 
flabellata, Hincks. 
Among about forty species of Hydroids that I have ob- 
served and collected in the White Sea, Obelia flabellata, 
Hincks, is very frequently met with. At the end of the 
month of June I found it with a great quantity of gonothece, 
all filled with young Medusz in various stages of development. 
Although in all other respects the Obelia flabellata of the 
* Hincks, Mon: Brit. Hydr. Zooph. pl. xlv. fig. 2, p: 231, 
t+ G. Sars, “Bidr. til Kundsk. om Norges. Hydroider,” Forh. i Vidensk, 
Selsk. i Christiania, 1873, p. 109, pl. iv. figs. 5-8. 
17# 
