382 



MEDUSA OF THE WORLD. 



Rhopalonema clavigerum. 



Plate 49, figs. I to 1". 



Marmanema clavigerum, Haeckel, 1879, Syst. der Medusen, p. 263, taf. 17, fign. I, 2. — Maas, 1893, Ergeb. der Plankton 

 Expcd., Bd. 2, K.c, p. 13. 



The bell is hemispherical, with a bluntly-rounded, apical projection, and about 7 mm. 

 wide. 16 tentacles, 8 of which are radial and are 2 to 3 times as long as the bell-diameter, with 

 swollen, tapering, club-shaped, ciliated outer ends. The 8 interradial tentacles are mere 

 slender, short, filiform cirri about half as long as the bell-radius. 8 lithocysts close to the sides 

 of the 8 interradial cirri. Viewing the medusa from the oral side so as to look into the bell- 

 cavity, the lithocysts are always in the direction in which the hands of a watch would move 

 from the interradial cirri to the lithocysts. Each lithocyst is a closed vesicle, within which 

 we find a club with a narrow stalk, and this club contains a single spherical concretion. 



The velum is very wide and muscular and swings to and fro within and without the bell- 

 cavity as the medusa swims. In common with other species of Rhopalonema, the medusa 

 gives a series of very rapid, powerful pulsations alternating with periods of complete rest. 



Fig. 225. — Rhopalonema clavigerum, after Haeckel, in System der Medusen. 



Stomach about as long as depth of bell-cavity; 4 short, simple lips. 8 spindle-shaped gonads 

 are developed upon the proximal halves of the 8 radial-canals adjacent to base of stomach. 



The whole medusa is of a glassy transparency, rendering it all but invisible in the water. 



It is widely distributed over the tropical Atlantic, from the Canary Islands to Tortugas, 

 Florida. The Plankton Expedition found it in abundance in the South Equatorial Stream. 

 It is common upon the surface. 



The position of the gonads distinguishes it from R. velatum, wherein these organs are upon 

 the middle thirds of the radial-canals, whereas in R. clavigerum they are adjacent to the base 

 of the stomach. 



Genus SMINTHEA Gegenbaur, 1856. 



Smitithea, Gegenbaur, 1856, Zeit. fiir wissen. Zool., Bd. 8, p. 245. — Metschnikoff, 1886, Arbeit. Zool. Inst. Wien, p. 244. 

 Trachynema, Maas, 1893, Ergeb. der Plankton Expedition, Bd. 2, K.c., p. 12. 



The type species is Sminthea eurygaster Gegenbaur, 1856, of the Mediterranean and trop- 

 ical Atlantic. 



GENERIC CHARACTERS. 



Trachymedusae with only 8 tentacles, one at the foot of each of the 8 radial-canals, 

 other respects this genus is similar to Rhopalonema. 



In 



