RHIZOSTOM^ — CASSIOPEA. 641 



1 Ins medusa was found m larj;e swarms by Keller on the shallow coral Hats of the southern 

 pans ot the Red Sea. It differs from the typical C. andromeda in the thick-rimmed sucker of 

 the exundirella, and the long, laterally compressed arms. Keller describes it in detail with good 

 figures. In common with other Cassiopeidas it lies upon the bottom with its oral surface 

 uppermost. Keller draws comparisons between its habits and structure and those of actinians, 

 etc. This medusa is probably only a local variety of C. nndroDicda, but the thick, sucker- 

 like disk at the middle of the e.xumbrella appears to distinguish it. 



Cassiopea xamachana R. P. Bigelow. 



Phue 69, fijis. 4 to S; platen 70 and 71; plate 72, the seven lower figures. 



Cassiopea xamachana, Bigelow, R. P., 1892, Zoolog. Anzeiger, Bd. 15, p. 212; Johns Hopkins I'niversity Circulars, 1892, 

 vol. I !, pp. 71, 84; 1893, Journal Institute of Jamaica, vol. I, p. 301, 1 plate; 1900, Memoirs Boston Soc. Nat. Hist., vol. 5, 

 No. 6, p. 191, figs. A to L, plates 31 to 38, 66 figs. — Perkins, 190^, Year Book of the Carnegie Institution, No. 4, p. 118. 

 Publication, No. lo2, p. 150, plate 4. — Mayer, 1906, Year Book of the Carnegie Institution, No. 4, p. 117; Publication 

 of the Carnegie Institution, No. 47, 62 pp. (rhythmical pulsation); 1907, Year Book Carnegie Institution, No. 6, p. 121; 

 Ibid., 1908, No. 7, p. 123. — Maas, 1903, Scyphomedusen dcr Siboga Expedition, Monog. 11, p. 40. — Stockard, 1907, 

 Y\'ar Book Carnegie Institution of Washington, No. 6, p. 119 (regeneration); Ibid., No. 7, 1908, p. 130. — Papers from 

 Tortugas Laboratory of Carnegie Institution of Washington, vol. 2, p. 61, figs. 1-29; Journal of Experiinental Zoology, 

 1909, vol. 6, p. 433, 8 figs. — Zeleny, C, 1907, Journal Experimental Zoology, vol. ^, p. 265, 4 text-figs, (regeneration) — 

 Dahlgren and Kepner, 1908, Text-Book of Principles of Animal Histology, p. 88, fig. 85 (histology of muscles). — Mayer, 

 1908, Papers from the Tortugas Laboratory of the Carnegie Institution of Wasiiington Publication, No. 102, p. 113 (the 

 cause of rhythmical pulsation); Popular Science Monthly, vol. 73, pp. 481-487, 4 figs.; 1909, Report of 7th Interna- 

 tional Zoological Congress, 4 pp. — Harvey 1909, Year Book of the Carnegie Institution of Washington, No. 8, p. 129. 



Cassiopea frondosa, FEV^fKES, 1882, Bull. Museum Comp. Zool. at Harvard College, vol. 9, p. 254, plate I, figs. 7-19; plate 2, 

 figs. 1, 2; plate 3, figs. 1-3, 9, 10; Ibid., 1883, p. So, plate I, fig. 16. 



The disk is usually about 150 mm. in diameter, although Bigelow records one from 

 Jamaica 240 mm. wide. It is flat and with rounded edges. There is a well-marked con- 

 cavity at the middle of the exumbrella, the diameter of which is about equal to the disk- 

 radius. It enables the medusa to cling firmly to any smooth surface as by a sucker. 



The number of the marginal sense-organs ranges from 1 1 to 23, although there are usually 

 about 16. For example, in 25 medusae taken at random and ranging in size from 23 to 149 

 mm., one had 13 marginal sense-organs, i had 14, i had 15, 12 had 16, 5 had 17, i had 18, 

 2 had 19, 2 had 20. I have seen one medusa with 11 and one with 23 sense-organs. The 

 number is independent of the size of medusa, being determined at time of strobilization. 



The sense-organs are short, blunt, and club-shaped, and are set within niches protected 

 above b\' a shelf-like membrane spanning the cleft between the adjacent lappets. There is 

 no exumbrella pit above the club. Each sense-organ contains a terminal mass of entodermal 

 crystals and an aboral cup-shaped ectodermal ocellus having reddish-brown pigment. There 

 are 5 short, blunt, rounded lappets between each successive pair of sense-organs; the 2 lappets 

 adjacent to the sense-organs are only about half as wide as the others. The mouth-arm disk, 

 which projects as a flat plate from the center of the subumbrella, is only about two-thirds as 

 wide as the disk-radius. 4 pairs of adradial mouih-arms arise from this disk. Each of these 

 8 mouth-arms is about 1.25 times as long as radius of bell and projects somewhat beyond 

 bell-margin. These mouth-arms are triangular in cross-section, their aboral surface being 

 broad and flat; the\- each give rise to 10 to 15 alternate, primary branches, which in turn 

 give rise to secondary branches. These branches are commonly longer than in C. frondosa 

 and are also longer and stouter than in 6'. andromeda Eschscholtz and more slender and have 

 more primary branches than in C. polypoides Keller. In the axil of each primary branch of 

 the mouth-arms is a single, flat, ribbon-like filament, which varies in length with the size of 

 the adjacent branch. There are also 5 to 13 large, ribbon-shaped filaments upon the oral 

 surface of the mouth-arm disk. The largest filament is at the center and is fully one-fourth 

 as long as the bell-diameter. The filaments decrease successively in length out over the 

 mouth-arms; those at the tips of the arms being only about one-seventh as long as those at 

 the center. In addition to the filaments there are numerous short club-shaped, nematocyst- 

 bearing vesicles scattered among the mouths. 



The mouths are found upon the oral (uppermost as the medusa lies upon the bottom) 

 sides of the primary and secondary branches of the mouth-arms, and to some extent upon 

 the oral sides of the 8 basal trunks of the arms. There are no mouths at the center of the 



