TRACHYMEDUS.E — RHOPALONEMA. 



379 



have been found at great depths, excepting by Chun in the Mediterranean. Hargitt found it 

 in the Gulf Stream off Woods Hole, Massachusetts. It is well known that the Mediterranean 

 is warmer (12. 6° to 13. 6° C.) at great depths than is the Atlantic or Pacific and this may 

 readily account for the peculiar distribution of R. velatum in this sea. This species is distin- 

 guished by its small gonads restricted to the middle thirds of the radial-canals and its short 

 stomach. The red color of the tentacles is not so distinctive, being quite variable in hue, or 

 even wanting. (See text-figure 218.) 



Rhopalonema umbdicata = Calyptra umbilicata Leuckart, 1856, is probably identical with 

 R. velatum. It is found in the Mediterranean and the tentacles are whitish, not red; also, the 

 manubrium is apparently as long as the depth of the bell-cavity, but the bell is represented as 

 expanded. VanhofFen, 1902, and Maas, 1905, regarded it as being identical with R. velatum. 



Rhopalonema typicum Agassiz and Mayer, 1902, from the eastern parts of the tropical 

 Pacific, is doubtless identical with R. velatum. It is larger than the typical R. velatum, being 

 15 mm. wide, and there is a well-developed apical projection at the center of the exumbrella. 

 There are about 16 lithocysts and 32 tentacles as in R. velatum; and the length of the stom- 

 ach and position of the gonads on the middle of the radial-canals are identical with R. 

 velatum. The tentacles are milky in color, not red as in the typical R. velatum. We may 

 consider it to be a large, relatively colorless variety of R. velatum. It is very abundant in 

 the tropical Pacific. (See text-figure 219.) 



The development has been studied by Gegenbaur, 1854, and MetschnikofF, 1886. The 

 egg is 0.24 mm. in diameter and is discharged into the water during the daytime. It remains 

 suspended in the water, neither sinking nor rising. Segmentation is total and equal and a solid 

 morula is formed, the central cells of which become entoderm, while the outside layer forms 

 the ectoderm. 



Gegenbaur, 1854, probably had the young medusa of this species, calling it " Trachynema 

 ciliatum." When I mm. wide there were 4 ciliated, radially placed tentacles alternating with 

 4 interradial sensory-clubs. The whole surface was ciliated and there were but 2 lips, these 

 being 180 apart. The second pair of lips do not develop until after the larva has 16 tentacles. 

 They then develop, forming a cross with the first pair. (See text-figure 213.) 



It is interesting to see that MetschnikofF finds that the young medusa of Aglaura hemi- 

 stoma has also but 2 lips 180 apart. 



A larval Cunina is often found attached to-the subumbrella of R. velatum at Naples, Italy. 



Rhopalonema polydactylum Haeckel. 



Rhopalonema polydactylum, Haf-CKel, 1879, Syst. der Medusen, p. 265, taf. 17, fign. 7-1 1. 

 Homaronema elongatum, Maas, 1893, Ergeb. der Plankton Exped., Bd. 2, K.c, p. 20. 



Bell pyriform to conical, with a narrow, rounded apex. It is 10 mm. wide, 8 mm. high. 

 32 tentacles of unequal length. The 16 larger, radial and interradial tentacles are as long as the 

 bell-radius; their outer ends are club-like and their epithelium is covered with cilia. The 16 



217. 



Fig. 216. — Rhopalonema velatum. Young medusa 

 showing the free sense-club not yet inclosed 

 within a capsule. From life, by the author, 

 Naples Zoological Station. Showing an enlarged 

 view of the naked sense-club. In later life 

 this club becomes partially inclosed within an 

 open crater-like capsule. 



Fin. 217. — " Sminthea globosa," after Gegenbaur, in 

 Zeit. fiir wissen. Zool., Bd. 8 = An immature 

 stage of Rhopalonema velatum. 



