BROWNE— MEDUSA FROM THE INDIAN OCEAN 193 



29. Ehopalonema velatum, Gegenbaur, 1856. 



Rhopalonema velatum, Maas, 1893, p. 14, Taf. 1, figs. 5, 9 — 11. 



Bhopalonema velatum, Vanhbffen, 1902, p. 59, Taf. 10, fiig. 16^ Taf. 11, fig. 32. 



Rhopalonema velatum,, Lo Bianco, 1904, p. 55, Taf. 34, fig. 137. 



Rhopalonema velatum, Maas, 1905, p. 50, Taf. 10, fig. 69. 



Rhopal,onem,a velatum, Bigelow, 1909, p. 129. 



Rhopalonema velatum, Mayer, 1910, p. 378, text-figs. 214, 216, 218, 219. 



Rhopalonem,a velatum, Vanhofien, 1912, p. 29. 



Rhopalonema velatum, Vanhofien, 1912, p. 371. 



Localities. Chagos Archipelago ; Mauritius ; Farquhar Group ; Amirante Isles. (The 

 collection contained about sixty specimens taken at about twenty different stations.) 



The specimens collected by the "Sealark" are about 2 to 5 mm. in diameter, and 

 in different stages of development ; the larger specimens have quite ripe gonads. The 

 gonads form elongated sweUings, not exceeding one millimetre in length and are usually 

 situated about the middle of the radial canals. In shape these gonads are very similar 

 to those of Rhopalonema velatum from Naples. 



Rhopalonema velatum and Rhopalonema coeruleum, have one character in common, 

 namely, a conical top-knot on the summit of the umbrella. Bigelow found the top-knot 

 to be very constant in his Pacific specimens. Although, at first, the top-knot was 

 not regarded as of any importance, later its absence became useful for distinguishing 

 Rhopalonema funerarium-, Vanhofien, from the other species of the genus. Nearly all 

 the specimens in the " Sealark " collection show clearly the top -knot on the summit 

 of the umbrella, but a few do not. The top-knot when properly formed stands out 

 as a kind of conical projection on the top of the umbrella, and is usually marked off 

 from the rest of the umbrella by a transverse circular furrow or depression. In shape 

 and size it varies considerably, and is scarcely recognisable in extreme cases, as only 

 a slight depression in the contour of the upper part of the umbrella is present. In this 

 collection there are a few specimens which show no signs of a depression. These specimens 

 are about 5 mm. in diameter, with well-developed gonads, and have rather a conical -shaped 

 umbrella. Although certain specimens have not the characteristic top-knot, still there 

 is not sufficient evidence to connect them with Rhopalonema funerarium. This latter 

 species has a differently shaped umbrella and apparently does not begin to develop its 

 gonads until about 6 mm. in diameter, and the gonads extend over the outer two-thirds 

 of the radial canals. It lives at a greater depth than Rhopalonema velatum, and belongs 

 to the mesoplankton. 



According to Vanhoffen Rhopalonema velatum has eight perradial tentacles, eight 

 interradial cirri, sixteen adradial cirri, and eight sense organs adjacent to the interradial 

 cirri. Certain authors, however, have described and figured Rhopalonema velatum with 

 sixteen sense organs. Maas (1893) gives the number of sense organs as sixteen, and 

 figures two octants with three sense organs in each, thus showing that the immber may 

 even exceed sixteen. Mayer (1910, p. 380) gives a figure of Rhopalonema velatum 

 drawn by himself at Naples, and it shows clearly sixteen sense organs, eight adjacent 



SECOND 'SERIES— ZOOLOGY, VOL. XVIL 25 



