Form oj Freshwater Ccelenterate. 123 



stomacli, the walls of which, as has been remarked, are formed 

 bj the annular fold of the endoderra. 



The gastrovascular space, dilated laterally into two sym- 

 metrical pouches, is continued into the hollow tentacles. In 

 these there are no cartilage-cells, but endodermal elements, 

 which often coalesce and, as it were, form a transition towards 

 cartilage elements. 



In a word, we have in our Polypodium a Hydroid organism, 

 the motile " trophosome " (B) of which, after several asexual 

 generations differing in the form and number of the tentacles, 

 becomes (as may be supposed) converted into the sexual 

 animal, the planula of which penetrates into the ovum of the 

 Sterlet, and living there as a parasite, gradually develops 

 into the stolo (A) with all its primary and secondary buds. 

 Here therefore a complicated metagenesis must be assumed, 

 during which the forms B, B', B", &c., up to the sexual 

 animal, perform the parts of nutritive and nurse-generations of 

 a still unknown cell-complex (of ecto- and endodermal origin), 

 which, in the last form (C) , becomes the place of formation of 

 the sexual organs. 



It would not surprise me at all if the club-shaped polypoid 

 of the grandchild-generation (B", fig. 13) were to become con- 

 verted into a Medusoid sexual form, and, indeed, by the out- 

 growth of the lower part of the crown of the club-shaped body, 

 by an annular fold, into a small bell with four marginal fila- 

 ments and two lateral tentacles, when the gastral cavity would 

 become more sharply separated into a ring- and four radial 

 canals. 



EXPLANATION OF PLATE IV. 



[All the drawings made from living specimens. Statement of 

 enlargements only approximate.] 



Fig. 1. An infected ovum, with a spirally twisted parasite in its interior 

 (stolo with buds). X 5. 



Fig. 2. A similar ovum, seen from one pole. Xo. 



Fig. 3. Stolo, with sixteen primary buds. X4. 



Fig. 4. A group of secondary buds in union with the stolo. X 5. The 

 introverted tentacles are seen shining through. Their arrange- 

 ment at the side of the buds is shown in the second bud. 



Fig. 5. Stolo bearing thirty-two developed buds with everted tentacles, 

 taken from the ovum of the Sterlet at spawning-time. X 3. 



Fig. (i. Two secondary buds at the time of the breaking up of the stolo 

 and formal ion of the mouth. Some of the lateral and basal 

 tentacles are retracted. X 15. 



Fig, 7. Mother-generation (B) of Polypodium hydriforme furnished with 

 twenty-four tentacles. x9. Twelve tentacles (4 -|- 2 -|- 4-1-2) 

 originate on the sides, and twelve (4-|-24-4-|-2) below. The 



