114 Dr. M. Ussow on a new 



Of tlie twentj-four tentacles (figs. 7 and 8) eiglit subse- 

 quently become differentiated (four above, I'.-IV'., and two 

 on each side of the bud, I.-IV.) as shorter, but at the same 

 time stouter, protractile tentacles, clavately enlarged at the 

 end, which I shall name feelers [Senhtastev) . At their 

 extremities they are provided with numerous urticating 

 elements, developed in special cnidoblasts, and they have the 

 appearance of filiform, clavate, simple structures without any 

 special armature. 



The other sixteen tentacles are placed in pairs symmetrically 

 on the two sides of the bud ; they are thinner^ but consider- 

 ably longer than the feelers. Further on, in the descrip- 

 tion of the free-living mother-form, the different functions 

 of these tentacles will be mentioned. 



At the commencement of the spawning-season, in the first 

 half of May, a great part of the buds have already turned out 

 the tentacles (fig. 5) ; but it is to be noted that even on the 

 buds of the same stolo this process by no means takes place 

 simultaneously. This delay in the development may fre- 

 quently be observed in the whole of the infected ova of a roe ; 

 then the parasites become soft and apparently perish. Their 

 colour in this case passes into a greenish tint, while the yolk 

 surrounded by them acquires an abnormal yellowish-red 

 coloration. The period at which the tentacles are everted 

 forms a turning-point in the existence of our animal. From 

 a sluggish parasitic form enclosed in a narrow space within 

 the envelopes of the ovum of the Sterlet there originates an 

 active generation ; the stolo with the well-developed buds 

 begins to move, and in consequence of this there frequently 

 arises, even in the interior of the Sterlet before spawning, the 

 possibility of a rupture of the egg-membranes, which are very 

 thin at this time, and of the liberation of the parasite. To 

 this liberation, however, the friction of the Sterlet during the 

 spawning, and in fact the whole spawning-process, is particu- 

 larly favourable. 



Whilst at the beginning of its • development, residence in 

 fresh water, even of short duration, was destruction to the 

 parasite, it cannot now develop itself further without water. 



If we examine an infected ovum during the spawning- 

 period, we no longer find any yolk in it outside the parasite, 

 but instead of it the remains of a brown excretion-product of 

 the animal, and the whole of the yolk not assimilated by the 

 buds has passed into their interior space, by which a change 

 of their colour to yellowish is j)i"oduced. 



In the course of a further sojourn of twenty-four hours in 

 the water, the entire stolo breaks up into thirty-two pieces, 



