92 OBELIA COMMISSURALIS. 



twelve and even sixteen Medusaa. The young Medusa (Fig. 135), when 

 hatched, has sixteen tentacles, four chymiferous tu.bes, a rather long 

 cylindrical digestive cavity, with four labial lobes ; there are no ovaries 

 yet developed. I have not found these Medusae in a more advanced 

 condition, though they become free in the first weeks of July, and are 

 found during the whole summer, as late as September, 

 but in no case were there any ovaries developed. In 

 confinement they do not prosper, and after a few days 

 die, without assuming a different shape from that in 

 which they become free. The tentacles are slender, as 

 long as the diameter of the disk ; in two of the tentacles 

 there are large marginal capsules in a swelling on the under side ; the 

 re-entering sjDur of the tentacles is small. There is considerable differ- 

 ence between the Hydrarium of the specimens found at Charleston and 

 those of our coast ; the Charleston specimens are uniformly thinner 

 and more slender ; it remains yet to be seen whether any further spe- 

 cific differences can be detected in the Meduste. If Van Beneden's 

 figure of the Oamjoanularia geniculata is correct, the European and the 

 American species of Obelia are distinct. 



Absecom Beach (Leidy) ; Charleston (McCrady) ; Buzzard's Bay, 

 Naushon (A. Agassiz) ; Massachusetts Bay and Grand Manan (Agassiz). 



Cat. No. 65, Charleston, S. C, January, 1852, L. Agassiz. Hydrome- 

 dusarium. 



Cat. No. 66, Charleston, S. C, February, 1852, L. Agassiz. Hydrome- 

 dusariura. 



Cat. No. 67, Nahant, July, 1861, A. Agassiz. Hydromedusarium. 



Cat. No. 68, Nahant, July, 1861, A. Agassiz. Hydromedusarium. 



Cat. No. 69, Nahant, July, 1861, A. Agassiz. Hydromedusarium. 



Cat. No. 70, Nahant, Sept. 1861, A. Agassiz. Hydromedusarium. 



Cat. No. 71, Grand Manan, Sept. 1857, J. E. Mills. Hydromedusarium. 



Cat. No. 72, Nahant, July, 1857, L. Agassiz. 



Cat. No. 73, Newport, R. I, Dr. Leidy. li^L^lv 



Cat. No. 392, Nahant, July, 1862, A. Agassiz. ^ 



Fig. 135. Quarter-disk of the Medusa of Obelia commissuralis. 



