804 SUMMARY OF CURRENT RESEARCHES RELATING TO 



was corroborated two years later by 0. Grimm.* The form described 

 has turned out to be a stage in the development of a free-living 

 hydroid organism. For the last two years this indubitably Ccelente- 

 rate form has been the subject of Dr. M. Ussow's researches, of which 

 only a preliminary notice has as yet been published. Eelated as this 

 form imdoubtcdly is to the Hydromedusas, the peculiarities which it 

 exhibits seem to warrant the invention of a special title — Pulypodium 

 hydriforme. The life-history of Polypodium is divisible into three 

 stages, — first, as a parasite in the eggs of Acipenser ruthenus in the form 

 of a cylindrical, spirally twisted tube with numerous lateral buds ; 

 second, as a free-living form, equipped with 24, 12, or 6 tentacles, 

 and dividing itself very frequently ; and third, presumably as a sexual 

 animal. 



After noting the extent, diagnosis, results, &c, of the parasitism, 

 Dr. Ussow describes the parasite itself. The youngest specimen 

 observed had the form of a cylindrical, hollow tube, 15-17 mm. in 

 length, 1^—2 mm. in thickness, and superficially beset with primary 

 buds. The walls consist of single layers of ectoderm and endoderm 

 and of spindle-shaped (mesoderm) cells between. As this muscular- 

 layer developes, the body becomes spirally coiled in the longitudinal 

 axis of the sturgeon's egg. The primitive buds become pear-shaped, 

 and the axial cavity of the organism is continued into each bud. 



Each of tho primitive buds soon exhibits a gradually deepening 

 furrow, dividing it into two pear shaped bodies — the secondary buds. 

 These are afterwards developed into free-living forms. The secon- 

 dary buds come, in consequence of spiral twisting, to lie on one side of 

 the whole organism (" stolon "), on that turned towards the chorion of 

 the egg. The ectoderm cells next the central yolk are filled with yolk- 

 granules, which they have directly ingested. The yolk-substance thus 

 acquired penetrates through the endoderm into the cavity of the buds 

 and accumulates as reserve material. 



The upper portion of the secondary bud exhibits a shallow 

 furrow, and represents the lower aboral end of the future free-living 

 form ; and the furrow extending parallel to the long axis indicates the 

 direction of a division which results in the halving of the free gene- 

 ration (or " mothers "). 



Tentacles are developed, as invaginate tubes, and exhibit all the 

 three layers. Of the 24 tentacles, eight are specially differentiated, as 

 short, strong, terminally swollen " Senktaster." They exhibit nume- 

 rous stinging cells developed in special cnidoblasts. The other 16 are 

 symmetrically arranged in pairs on both sides of the bud ; they are 

 thinner and much longer than the other eight. The tentacles are 

 gradually and irregularly evaginated, the stolon begins to move, and 

 eventually effects its liberation during spawning. 



After being in water for 24 hours or so, the whole stolon falls into 

 32 pieces, representing the 32 buds ; and this disruption occurs in a 

 perfectly definite fashion. The buds have changed their form con- 

 siderably since their first formation, and after liberation the old 



* Arbeit. Naturf. Gesell. Petersburg, 1873, Taf. ii. 



