NOTE ON THE OCCURRENCE OF CASSIOPEA XAMACHANA 

 AND POLYCLONIA FRONDOSA AT THE TORTUGAS. 



(Plate 4, figs. 17-20.) 



Amongst the various forms of plants and animals which find a conven- 

 ient and salubrious abode in the warm storm-proof waters of the Fort Jef- 

 ferson moat, none is more characteristic than the rhizostomous scyphozoan 

 medusa Cassiopea .raniachana Bigelow. The favorable conditions which 

 Bigelow 1 found to prevail in the Salt Ponds of Jamaica must have been 

 very much the same as those which are so marked in the sheltered moat 

 in the Tortugas. 



Besides the large bronzy-black ascidians that grow upon the rock walls 

 of the moat at tide-mark, no creature is so conspicuous to the eye of the 

 zoologist as the feathery brown disks that fairly carpet the floor of this place. 

 When the surface of the water is unruffled, these jelly-fishes can be 

 counted by the hundred as they lie on the warm sand or amongst the masses 

 of algse, the fluffy branches of the oral arms uppermost, the edge of the 

 disk lazily fanning at the rate of a few strokes to the minute. " Moss 

 cakes " the marines at the fort called the great creatures. Judging by both 

 size and numbers, this species has here found an ideal breeding-ground. 2 



The medusae vary in size through a wide range, and the extremes are as 

 apt as not to be found resting side by side on the sand. 



The largest examples measured 145 to 155 mm. in diameter, and there 

 were very many of this size. The smallest specimens were less than 25 

 mm. in diameter, and they were characterized by less distinct markings, 

 oral arms of smaller proportionate size, and greater activity of habit. The 

 parts of the moat where the bottom was composed of clean sand, with only 

 a fathom of water, seemed most favorable to the small individuals. In 

 these younger cassiopeas the number of marginal sense-organs was from 

 13 to 15, while the largest and oldest ones possessed from 18 to 22. 



Sexual multiplication. Very little is known about the reproductive pro- 

 cesses of the rhizostome medusae. Bigelow, in his admirable monograph 

 on this species, has given us a most entertaining as well as thorough account 



'Bigelow, R. P. 1900. Memoirs Boston Soc. Nat. Hist., vol. 5, No. 6. Anatomy 

 and development of Cassiopea .ramachana. 



2 The first record of the occurrence of this species in this locality is given by 

 Fewkes, J. Walter, 1882: Notes on Acalephs from the Tortugas (Bull. Mus. Comp. 

 Zool. ix, 7). While his determination of the specimens found at Fort Jefferson was 

 as Cassiopea frondosa, his description and figures, and the occurrence of C. .vamachana 

 in the same locality, make it clear that Bigelow was justified in assuming that the 

 species was in fact C. xamachana. 



150 



