140 Papers from the Marine Biological Laboratory at Tortugas. 



warrant us in holding such fortuitous variation responsible for the occur- 

 rence of large communities of the genus, in which so many individuals show r 

 a definite numerical arrangement. Perhaps that is a matter of opinion. 



The first discovery of the genus, and the later study of it that has 

 been carried on by several different observers, has had the peculiarity of 

 depending upon aquarium material. Dujardin, in 1843, found the first 

 medusa and later the hydroid in an aquarium stocked with material from 

 the coast of France. The eggs of the genus must be capable of extended 

 travels, judging by the great distances separating the localities where the 

 hydroids have become established. It was doubtless in the egg stage, or 

 possibly as a free planula larva, that the species was introduced into the 

 aquarium where Dujardin discovered it. The other localities where rep- 

 resentatives of the genus have been found are as follows: Brittany, Belgium, 

 Messina in the Mediterranean, and the Bahama Islands. This latter hab- 

 itat is so near the Tortugas that it might be expected that the two related 

 species found in these neighboring localities would show closer similarity 

 than two which occurred at a greater distance apart. Reference to the fol- 

 lowing table will show that this is not the case. The only point of agreement 

 between the two, as regards numerical arrangement, is in the gastric pouches. 

 Another point not indicated in the table is the difference between the 

 tentacle processes of the two species. In the Bahama form, 1 the prehensile 

 branches are developed at the expense of the floating nettling-threads. 

 whereas in the species from the Tortugas these terminal filaments are the 

 most conspicuous feature. The habitat of the species found in the Ba- 

 hamas is in open, exposed shallows with sandy bottom. The long, floating 

 filaments would increase the risk of the creature's being swept away by the 

 waves. Their reduction must be an advantage. 



Comparison of species of Cladoncma. 



Discovery of C. mayeri. The peculiar conditions of temperature, free- 

 dom from wave-action, and bottom-growth tempted me to investigate into the 

 ecology of the Fort Jefferson moat. Inasmuch as I was particularly inter- 

 ested in the ccelenterate fauna, I worked at first with a fine-meshed tow-net. 

 The first day this was tried some specimens of the medusa came into the net 



1 Perkins, 1902. Johns Hopkins University Circulars No. 21, Vol. xxi, No. 155. 



