110 Dr. M. Ussow on a neio 



XII T. — A new Form of Freshvater Coelenterate. 

 By Dr. M. Ussow *. 



[Plate IV.] 



The parasite of the eggs of the Sterlet, first described by P. 

 Owsjannikow t J^nd somewhat Later by O. Grimm J, consti- 

 tutes only a stage in the development of a Hydroid organism 

 living free in the Volga. 



Although the form of the body, the mode of life, the deve- 

 lopment, and the anatomical structure of this animal indubit- 

 ably indicate its coelenteric nature and approximate it to the 

 Hydromedusffi, the generic characters of the latter apply so 

 little to our organism, that it appears not unadvisable to give 

 it a new name, and I will call it Polypodmm hydriforme. 

 Since March 1884 I have occupied myself with the investi- 

 gation of this form, and I am preparing a large memoir for 

 the press ; at present I shall only notice in a few words the 

 anatomical structure and the more important vital pheno- 

 mena of this peculiarly interesthig animal. 



As regards its mode of life and development, we meet with 

 Folyiwdium in three stages, namely : — 1, as a parasite in the 

 eggs of the Sterlet {Aciiienscr ruthenus), in the form of a 

 cylindrical, spirally-twisted tube, furnished with numerous 

 lateral buds ; 2, free, frequently dividing, provided with 24, 

 12, or 6 tentacles ; 3, as I suppose, as a sexual animal. The 

 transformation of the free-living into the sexual form has not 

 hitherto been demonstrated (September 1885), not withstanding 

 the successful culture of the animal in the aquarium ; I must 

 therefore speak of it hypothetically, basing what I have to 

 say upon the first two developmental stages. 



As regards the disease of the eggs of the Sterlet, we may 

 remark as follows : — 1. The number of infected fishes is to 

 that of the healthy ones as 2 : 10. 2. The disease is depen- 

 dent on the age of the animals, and in large fishes of 50-70 

 centim. in length we find diseased ovaries more frequently 

 than in fishes of 20-25 centim. 3. Sterlets which have been 

 long kept in fish-barges harbour more parasites than freshly 



* Trauslated by W. S. Dallas, F.L.S., from the ' Morphologisches 

 Jalirbuch,' Eaml xii. pp. 137-153. 



t " Arbeiten der dritten russischeu Naturforscherversammlung in Kiew " 

 (ref. in Zeitsclir. f. wiss. Zool. Bd. xxii. p. 21)2), ' Melauges biologiques 

 de I'Acad. des Sci. de St. Petersb.,' 1871. 



\ ' Arbeiten der Naturforschergesellscliaft in Petersburg/ 1873, Taf. ii. 

 (Materialien zur Keniitniss d. nied. Orgaiiismen, 3. Dissertation). 



