252 M. C. Mereschkowsky on the Hydrovda. 
White Sea did not differ at all from that described by Hincks*, 
the gonotheca was distinguished by the absence of the little 
tubular elevation placed upon its flattened summit, of which I 
never observed any trace; nevertheless it can only be re- 
garded as a feeble arctic variety of the British Obelia flabel- 
lata. 
The development of the Medusez has been studied princi- 
pally by L. Agassizt, who was the first to publish some im- 
portant facts, and by F. E. Schultzet, as well as by Mr. All- 
man (the last on Corymorpha nutans), who have made some 
alterations in the views current before their time. But as most 
attention has been paid to the Meduse belonging to the order 
Athecata, I have thought that it might not be altogether with- 
out interest to have their results confirmed by a Medusa be- 
longing to the quite different order Thecaphora. 
Plate XIII. fig. 1 shows the first commencement of a Medusa, 
which only consists of a protuberance (expulsion) of the walls 
of the blastostyle, composed, like the latter, of two layers, the 
ectoderm and endoderm, separated from one another by a very 
thin layer, which is not so distinctly contoured as in the buds ; 
it is, no doubt, the hyaline intermediate layer between the 
ectoderm and endoderm which Schultze calls the “ Stiitzla- 
melle.” In form, this bud differs in no respect from a young 
hydranth developed from a planula; and both have exactly 
the same form as Protohydra Leuckartit, Greef, and the same 
as must have been possessed by Hackel’s Archhydra§, 7. e. if 
we choose to accept Hickel’s biogenetic law. The next 
stage is represented in fig. 2 (Pl. XIII.) ; we see that the bud 
has considerably widened, and that the ectoderm (which is 
here also distinctly separated from the endoderm by the double- 
contoured line) has become much thicker at the summit of 
the bud than elsewhere. The thick part, which is in the form 
of acone, is turned downwards towards the endoderm, in which 
the cone buries itself; the inner surface of the endoderm and 
also the general cavity of the body retain their original form : 
they do not form any expulsion; and their apex is always 
hemispherical. But, at the same time, we already remark 
that the depression of the outer surface of the endoderm is not 
a regular cone, but, on the contrary, that the edges of this 
depression are dentate. There are four teeth formed by the 
superior layer of the endoderm, and between these teeth four 
depressions occupied by the inferior layer of the ectoderm. It 
* ‘History of British Hydroid Zoophytes,’ i. p. 157. 
t ‘Contributions.’ . 
t ‘Ueber den Bau der Syncoryne Sarsit,’ 1873, p. 27. 
§ It is his form “ monaxonia diplopola inarticulata.” 
