378 



MEDUSiE OF THE WORLD. 



Rhopalonema velatum Gegenbaur. 



Trachynema ciliatum (young medusa), Gegenbaur, 1854, Zur. Lehre Generationswechsel bei Medusen, p. 52, taf. 2,fign. 17-23; 



1856, Zeit. fur wissen. Zool., Bd. 8, p. 249, taf. 9, fig. 6. 

 Rhopalonema velatum, Gegenbaur, 1856, Zeit. fiir wissen. Zool., Bd. 8, p. 251, taf. 9, fign. 1-5 (mature medusa).— Haeckel, 



1879, Syst. der Medusen, p. 264.— Maas, 1893, Ergeb. der Plankton Exped., Bd. 2, K. c, p. 14, taf. 1, fign. 5, 9-11 — 



1904, Result. Camp. Sci. Prince de Monaco, fasc. 28, p. 27; 1905, Craspedoten Medusen der Siboga Expedition, Monog. 



io, p. 50, taf. 10, fig. 69. — Vanhoffen, 1902, Wissen. Ergeb. deutsch. Tiefsee Expedition, Dampfer Valdivia, Bd. 3,Lfg. 



1, p. 59, taf. 10, fign. 16, 28; taf. II, fig. 32. — Metschnikoff, E., 1886, Embryol.Studien an Medusen, Wien, pp. 23 (egg), 



61 (segmentation and formation of entoderm), taf . 6, fign. 13-15. — Maas, 1906, Revue Suisse de Zool., tome 14, p. 97.— 



Bigelow, H. B., 1909, Mem. Museum Comp. Zool. at Harvard College, vol. 37, p. 129. 

 Trachynema oaonarium (young medusa), Haeckel, Ibid., p. 260. 



Marmanema tympanum (medusa with mouth lost through accident), Haeckel, Ibid., p. 262. 

 ( >) Cahptra umbilicata, Leuckart, 1856, Archiv. fiir Naturges., Jahrg. 22, p. 14, taf. I, fign. 9, lo=Marmanema umbilicatum, 



Haeckel, 1879, Syst. der Medusen, p. 263. 

 Marmanema velatoides, Maas, 1893, Ergeb. der Plankton Expedition, Bd. 2, K.c, p. 13, taf. 1, fig. 6. 

 Rhopalonema txpicum, Agassiz, A., and Mayer, 1902, Mem. Mus. Comp. Zool. at Harvard College, vol. 26, p. 152, plate 5, figs. 



21, 22.— Hargitt, 1902, Biol. Bulletin, vol. 4, p. 14, fig. 2; 1904, Bull. U. S. Bureau of Fisheries, vol. 24, p. 54, 1 fig. 

 (?) Sminthea eurygaster (young medusa), Metschnikoff, E., 1886, Arbeit. Zool. Inst. Wien., Bd. 6, p. 244, taf. 22, fign. 17, 20. 



Bell hemispherical, often with a low, solid, dome-like apical projection, 8 to 15 mm. wide, 

 with thin but quite rigid walls. 8 straight, narrow radial-canals, 4 perradial and 4 interradial, 

 8 marginal tentacles at bases of the 8 radial-canals. These tentacles are twice as long as bell- 

 diameter and have enlarged, spindle-shaped distal ends. There are I to 3 smaller tentacles in 

 each octant between the radial-canals. These are less than one-fourth as long as the 8 principal 

 tentacles and are more like cirri with club-shaped ends. There are thus altogether 32 tentacles, 

 8 at the bases of the radial-canals and 24 (3X8) spaced at equal distances around the margin 

 between the radial-canals. All of the tentacles are very brittle and are usually broken off 

 short in specimens taken in a tow-net. They are all solid, their entoderm being composed of 

 chordate cells. 



In young medusae there are only 4 lithocysts, one in the middle of every alternate octant 

 between radial-canals, but when older there are 8 lithocysts, one near the middle of each 

 octant, and the lithocysts finally increase to 16, which follow in clock-wise rotation close to 



Fig. 213. — "Trachynema ciliatum" =»young of Rhopalonema velatum. 

 Fig. 214. — Rhopalonema velatum. 



Fig. 215. — "Sminthea tympanum," = Young stage of Rhopalonema velatum. 

 Above figures after Gegenbaur, in Zeit. fiir wissen. Zool., Bd. 8. 



the points of origin of the perradial and interradial tentacles. Each mature lithocyst is a 

 simple, closed, marginal vesicle containing a sensory-club with a single spherical concretion ot 

 entodermal origin. When young, however, the lithoeyst-club is naked and it is only later that 

 the ectodermal capsule grows outward from the bell-margin and forms a crater-like wall 

 around the club. 



Stomach small and short, only about half as long as depth of bell-cavity; octagonal above, 

 4-sided below; 4 simple, short lips in the radii of 4 of the radial-canals. The gonads are 8 

 linear to egg-shaped pouches restricted to the middle thirds of the 8 radial-canals. They begin 

 to develop when the medusa is about 4 mm. wide. 



Stomach and gonads milky-white; tentacles carmine, specially at their tips. 



This medusa was first adequately described by Gegenbaur and Haeckel from the Mediter- 

 ranean, where it is exceedingly abundant. Since then it has been found widely distributed 

 over the tropical Atlantic, Pacific, and Indian oceans. It is a surface form and appears not to 



