M. C. Mereschkowsky on the Hydroida. 339 
7. The medusa of Obelia flabellata is developed in the 
manner ascertained by F. HE. Schultze in the case of Syn- 
coryne Sarsit, in the first place by the impulsion of the active 
ectoderm into the passive endoderm. 
8. The first stages of the development of the ova of the 
medusa of Obelia flabellata before fertilization consist in a 
repeated division of the nucleolulus, followed by the division 
of the nucleolus into several parts, a division which stops at 
the nucleus. 
9. Obelia flabellata, under certain conditions, appears to be 
able to increase by spontaneous fission by a sort of cyst, after 
the fashion of Schizocladium ramosum and Corymorpha. 
10. There are more than forty species of Hydroids in the 
White Sea, about eight of which are new. The fauna is more 
polar than that of the north of Norway and the Mourmansky 
bereg, and shows some oriental features (7. e. features of the 
fauna of the Pacific Ocean). It does not prove Lovén’s 
hypothesis of a connexion between the White Sea and the 
Baltic. 
11. Oorhiza borealis, nov. gen. et sp., is distinguished by 
the sporosacs issuing immediately from the hydrorhiza with- 
out the intervention of blastostyles. 
12. There are Hydroids (Oorhiza borealis) the tentacles of 
which are furnished with eyes (or ‘ eye-pigment’’). 
13. The northern variety of Sertularella polyzonias must 
constitute a distinct species—Sertularella gigantea, mihi. 
14. Sertularia albimaris, new species, with a hydrorhiza 
in a continuous layer. Description of Leptoscyphus Grigo- 
riewt, NOV. sp. 
15. The forms of the family Sertulariide, which have their 
hydrothece arranged not in two but in several series, must 
form a new genus, Polyserias. (Description of two species.) 
EXPLANATION OF PLATES. 
Prats XII. 
Fig. 1. Young medusa of Obelia flabellata in the form of Archhydra. 
Figs. 2, 3. Subsequent stages of development, in which the ectoderm alone 
is active and buries itself in the endoderm. 
Fig. 4. Optical section of fig. 5. 
Fig. 5. The four radial canals, strongly developed ; commencement of the 
formation of the manubrium. 
Fig. 6. Young medusa still attached to the blastostyle. 
Fig. 7. Medusa of Obelia flabellata, completely developed and furnished 
with four sporosacs. 
Figs. 8, 9. 10. Ova taken from the sporosacs of the medusa of Obelia fla- 
bellata, not fecundated, and showing different stages of develop- 
ment of the nucleolus and nucleolulus. 
Fig. 11. The nucleus, highly magnified, to show the relative size of the 
