ZOOLOGY. 483 



its life history. In 1871 Dr. Owsjannikow made known a peculiar par- 

 asite discovered in the ova of the sturgeon. This parasite proves now 

 to be simply " a stage in the development of a free-living hydroid." It 

 was made the subject of investigation by Dr. M. Ussow for two years, 

 and the results of his researches have appeared in a preliminary notice. 

 The form in question is related to the hydromedusse in general, and 

 has been named Polypodium hydriforme. The life history of the newly 

 discovered hydromedusan is divisible into three stages, first, as a par- 

 asite in the eggs of the sturgeon, Acipenser rnthenus, wherein it exists 

 as a cylindrical spirally twisted tube, with numerous lateral buds; then 

 as "a free-living form, equipped with 24^ 12, orG tentacles," and finally, 

 "presumably as a sexual animal." 



The first stage, or that of parasitism, is noteworthy. " The youngest 

 specimen observed had the form of a cylindrical hollow tube, 15 to 17™°^ 

 in length, IJ to 2™™ in thickness, and superficially beset with primary 

 buds. The walls consist of single layers of ectoderm and eudoderm, 

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

 layer develops, 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 the 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 secondary 

 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 eudoderm into the cavit^^ of the buds and accu- 

 mulates as reserve material. 



"The upper portion of the secondary bud exhibits a shallow furrow, 

 and represents the 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 generation (or 'mothers'). 



" Tentacles are developed as invaginate tulles, and exhibit all the 

 three layers. Of the twenty-four tentacles, eight are specially differen- 

 tiated, as short, strong, terminally swollen 'Seuktaster.' They exhibit 

 numerous stinging cells developed in special cnidoblasts. The other 

 sixteen 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 twenty-four hours or so the whole stolon 

 falls into thirty-two pieces, representing the thirty-two buds; and this 

 disruption occurs in a perfectly definite fashion. The buds have 

 changed their form considerably since their first formation, and after 

 liberation the old stalk and an adjacent portion of the stolon form a 



