S Papers from the Marine Biological Laboratory at Tortugas. 



the lower tentacles, and the location of the gonophores considerably above 

 the latter, are points distinguishing the species from those described by other 

 observers. 



No specimens appeared in which there was any sign of a branching colo- 

 nial stock, such as is described as an occasional form of the trophosome of 

 C. aUinaiii 1 and C. dujardinii.- 



Habitat. Not found thus far in any other locality than that given above 

 for the medusa form, viz, the growing filamentous alga, abundant in the 

 Fort Jefferson moat. Owing to its small bulk and unobtrusive appear- 

 ance, it would be difficult to discover this hydroid upon any other than a 

 very delicate foundation. It may be abundant upon stones and shells, etc., 

 in the bottom of the moat, but it would be only by rearing the medusa; in 

 aquaria containing nothing else that one would be likely to find it there. 



III. GENERAL ACCOUNT OF THE MEDUSA STAGE OF CLADONEMA MAYERI. 



The occurrence in the moat at Fort Jefferson of this species of Clado- 

 nema, or of any species of the genus for that matter, is certainly surprising. 

 It would be difficult to find a part of the ocean more unlike the habitation 

 one would select as that for which the structure of this creature seems to 

 fit it. Here is a creature very unusually equipped for life in the open sea, 

 capable of resisting ocean-currents, tides, and boisterous waves, provided 

 with ballast and a whole battery of anchors against the assaults of tempests. 

 It has established itself in cowardly fashion within the sheltering walls of a 

 placid ditch, well out of harm's way. 



The moat at Fort Jefferson, surrounding the hexagonal fortifications, is a 

 relic of the ancient days of short-range artillery. It was constructed by 

 throwing a substantial wall of masonry around the vertical face of the fort, 

 founding it upon the natural bottom of coral rock. The moderate tides have 

 access through the generous sluiceways, built large enough to permit the 

 passage of small boats. Although shallow,, the moat is never empty, nor is 

 its bottom, which rises to within a few feet of the surface at low water, 

 ever entirely uncovered, even in the shallowest parts. The water is warmed 

 by the sun and by reflected heat from the shallow, sandy bottom and the 

 brick walls of the fort. The temperature of the water is often high enough 

 to make it feel decidedly warm to the hand when the air is well up in the 

 eighties, Fahrenheit. 



The ecology of the moat offers a most interesting problem. A number 

 of species have become established here which never make their appearance 

 in the waters outside the wall of the moat. The warmth and quietness of the 

 water is partly the cause of this condition, but there is also to be taken into 

 account the presence, because of that same warmth and protection from 



1 Allman, J. G., 1871. Monograph of Gymnoblastic Hydroids. 

 2 Dujardin. Ann. des Sci. Nat., 1843, p. 370. 



