RHIZOSTOM^ — CASSIOPEA. 637 



Tilesius, 1834, figured 4 species of Cassiopen and represented each of them as having 

 8 subgenital cavities. Relying upon the figures of Tilesius, L. Agassi/., 1862, separated the 

 genus Polyclonia, having but 4 subgenital cavities. Later researches have demonstrated 

 that all the known species of these medusae have normally but 4 subgenital cavities, and 

 should therefore be placed in the genus Cassiopea. Haeckel, 1880, attempts to separate 

 Casstopva from Polyclonia by calling medusae with 16 marginal sense-organs Cassiopca, 

 while those with 12 of these organs are called Polyclonia. The number of marginal sense- 

 organs is, however, very variable, not only among different species of these medusa but also 

 among individuals of the same species, and therefore can not be used as a means of establish- 

 ing generic distinctions. 



The medusae ot this genus are all inhabitants of warm oceans, and are found in greatest 

 abundance in the tropical coral regions of the East Indies and Red Sea. R. P. Bigelow finds 

 that C. xamachana from the West Indies develops through the monodiscus strobilization 

 of a scyphostoma and the young ephyras of this species and of C. frondosa have a simple, 

 central, 4-cornered mouth, thus recalling the adult condition in the Semaeostomes, from which 

 forms the Rhizostomae have evidently been derived. The rhopalia of the ephyra are derived 

 from the bases of each alternate tentacle of the scyphostoma, the other tentacles degenerating. 



The number of "species" of Cassiopca has been multiplied greatly, owing to the remark- 

 able color-range and variability in other respects of these medusae. These color types appear 

 to be local, and the Cassiopca medusae of almost every new region of the tropics are nearly 

 certain to be described as "new species" based on color peculiarities. It is therefore impos- 

 sible, at present, to classify the forms of Cassiopca with any degree of certainty. 



The blue and amber-green coloration of these medusje is due to the presence of com- 

 mensal plant organisms. Colasanti, 1886, describes the blue pigment matter as zoocyanin. 



Maas, 1903, attempts to separate the genus into two cohorts; one, consxsimgoi C.mertensi, 

 C. rnertensi var. ndrosia, pol\poides. xamachana, ornaia, and ornata var. digitata, distinguished 

 by its long, cylindrical, pinnately branched mouth-arms. The other group consists of C. 

 andromcda and its varieties: C. dcprcssa and C. deprcssa var. picta. This latter cohort has 

 irregularly branched, short, flat mouth-arms. An idea of the range in color-patterns of these 

 medusae may be obtained from an inspection of plates 70 to 72 which exhibit photographs of 

 a few of the varieties of Cassiopca xamachana, all taken in the course of an hour from the 

 moat of Fort fefferson, Tortugas, Florida. 



Stockard demonstrated that in C. xamachana the nearer the injury is made to the center 

 of the disk the more rapid the rate of regeneration. He also found that the more arms we 

 remove, the more does the central disk shrink during the growth of the regenerating arms, and 

 he thus finds that the regenerating tissue absorbs nutriment at the expense of the normal body 

 tissue as do cancerous tissues in their growth. 



Mayer finds that the rhythmical pulsation in C. xamachana is due to a nervous stimulus, 

 and this stimulus is caused by the presence of a slight excess of sodium in the rhopalia over and 

 above the concentration of this ion in the surrounding sea-water. This excess of the sodium 

 ion is due to the constant formation of sodium oxalate in the sense-club, and this oxalate 

 precipitates the calcium chloride of the sea-water to form the calcium oxalate crystals of the 

 sense-club and sets free sodium chloride. 



R. P. Bigelow finds that the vesicles between the mouths of C. frondosa serve to capture 

 prey and to thrust the food into the mouths. 



Cassiopea andromeda Eschscholtz. 



Medusa andromeja, Forskai., 1775, Descript. que in Itincre Orientali Observavit, Hauni.T, p. 107, tab 31,3 fign. 



Cassiopea andromeda, Eschscholtz, 1S29, Syst. dcr Acalephcn, p. 43.— Tilesiis, 1829, Nova Acta Phys. Men. N. C, tome 15, 

 p. 266, taf. 69, 70. — Milne-Edwards, 1849, Cuvier's Regne Animal lUustre, Zooph., plate 51, fig. l.— Haeckel, 1880, 

 Syst. der Meduscn, p. 569.— Keller, 1888, Zool. Anzeiger, Bd. 11, pp. 359, 389.— Hartlaub, 1909, Zoolog. Jahrbiichcr, 

 Abth. Syst., Bd. 27, p. 467, taf. 23, fign. 1-8. 



Cassiopea forskaica, Peron et Lesueur, 1809, Annal. du Mus. Hist. Nat., Paris, tome 14, p. 356. 



Bell flat, shield-shaped, 100 to 120 mm. wide, 20 to 30 mm. high. 15 to 18, usuall) 16, 

 marginal sense-organs. A variable number of short, blunt lappets. In each paramere are i to 

 6, usually 3, velar flanked by 2 ocular lappets. 8 mouth-arms, wide, flat, and hardly as long as 



