336 



MEDUSAE OF THE WORLD. 



This medusa occurs in large swarms in pure ocean-water from the southern coast of 

 Long Island to Beaufort, North Carolina, during summer and autumn. Mature individuals 

 are abundant off the New Jersey coast in October. It is not found in bays or in brackish 

 water, but is strictly a creature of the open sea. A large, relatively colorless variety of this 

 medusa ranges from Cape Cod to Greenland. The southern variety is distinguished by its 

 decided pink coloration, smaller size when mature, and in the fact that the tentacles are only 

 slightly more numerous than the radial-canals, while the lithocysts are very small and 8 to 10 

 times as numerous as the tentacles. The excretion papillae lack the green pigment observed 

 in the northern variety of Z. grcenlandica. 



Fig. 193.- 



'Zygodactyla crassa," after A. Agassiz, in North American Acalephx. 

 Side view of medusa. 



The southern variety of Z. grcenlandica may bear the same relationship to the northern 

 that the southern, pink-colored Cyanea versicolor bears to the brown-colored Cyanea arctua of 

 northern waters. 



Young medusa. — When about 3 mm. in diameter the bell is hemispherical in shape. 

 There are 4 broad, straight radial tubes and 4 others that extend but a short distance from 

 the periphery of the stomach towards the circular tube. There are 4 long tentacles at the 

 bases of the 4 complete radial tubes and also 4 rudimentary tentacles in the radii of the 4 

 incomplete radial tubes. There are 8 lithocysts. Velum well developed, manubrium wide and 

 shallow. The outer surface of the exumbrella is crossed by 4 radially situated rows of nemato- 

 cyst-cells. These diminish in size with the growth of the medusa and are not seen in the 

 adult. As development proceeds the tentacles increase more rapidly than the radial tubes. 

 New radial tubes develop from the periphery of the stomach and extend towards the circular 

 canal; at first they are very slender, but after reaching the circular tube they soon increase in 

 caliber. The tubercles of the floor of the subumbrella begin to develop when the medusa has 

 acquired 8 radial tubes; they first appear near the region of the periphery of the stomach. 



Zygodactyla (?) crassa A. Agassiz. 



Zygodactyla crassa, AfiAssiz, A., 1865, North Amer. Acal., p. 106, figs. 157, 159. 

 Polycanna crassa, Haeckel, 1879, Syst. der Medusen, p. 232. 



Adult medusa. — Bell hemispherical in form and about 300 mm. in diameter. About 350 

 long, marginal tentacles, more massive than in Z. grcenlandica. Only about 32 radial tubes. 



