228 On Hydroids from the White Sea. 



cylindrical, truncated above, with a single filiform tentacle, 

 placed higher than the middle of body. Gonophores without 

 blastostjles, medusiforra planoblasts; medusa with four radia- 

 ting canals, sixteen tentacles, and eight well-developed genera- 

 tive sacs, two from each radiating canal. 



MonohracMum parasitum^ nov. sp. 



Trophosome. Hydrocaulus about one fourth or one fifth of 

 the hydranth, formed by a chitinous tube springing from the 

 hydrorhiza, which covers the end of the Tellina-solidula shell. 

 Hydranth 2 millims. in length, cylindrical, without a hypo- 

 stome, truncated at the summit, which is provided with a 

 simple, regular, round aperture. 



The single filiform tentacle very long, from three to four 

 times the length of the body, is placed at the upper half of the 

 body at a small angle. 



Gonosome. Stem of the gonophore short and thin. The 

 medusa has eight generative sacs of the same length as the 

 bell. 



In one of the next numbers of this Journal I will describe a 

 few more equally interesting genera and species of Hydroids 

 from the northern seas of Russia. Thus, for instance, I have 

 two Hydroids from the White Sea which belong to the order 

 Thecaphora (family Sertularids) , of which I form a separate 

 genus {Polyserias) ; their hydrothecffi are arranged, not as 

 usual in two, but in six or even more rows. One of them 

 (PL VI. figs. 15, 16) I have named Polyserias Hinchsn, in 

 honour of the Rev. T. Hincks, whose writings on the Hydroida 

 are deservedly held in high estimation ; another species 

 of the same genus, of which the hydrothecas are joined to the 

 stem as in Thuiaria.^ I naiue Polyserias glacialis. I found 

 several other species of this genus in the collection of Hydrdids 

 in the St.-Petersburg Museum of the Academy of Sciences, 

 brought from the Sea of Ochotsk and Kamtschatka. Another 

 interesting genus, also from the White Sea, was found by 

 Professor Wagner of the St.-Petersburg University, to which 

 I have given the name Oorhiza. It is distinguished by having 

 sporosacs with only one q^^^^ rising from the hydrorhiza without 

 the aid of a blastostyle, by which it differs from Hydractinia^ 

 which always has blastostyles. In all else there is very 

 little difference from Hydractinia or Podocoryne ; therefore it 

 belongs to the family Hydractiniidse. A species of Sertu- 

 laria is interesting, from having its hydrorhiza continuous, 

 formed by the confluence of the separate threads. I shall 

 name this species, which is also from the White Sea, 

 Sertularia albimaris. 



