144 Papers from the Marine Biological Laboratory at Tortugas. 



Specific gravity.- The reason for the apparent sluggishness of the 

 medusa, mentioned above, is not far to seek. The tentacles are so laden 

 with concretions that they must be a good deal of a burden to the small 

 creature. The center of gravity is so far down on the bell and tentacles 

 that a specimen, inverted in the \vater and released, will right itself at once. 

 That this is not a muscular act is seen in experiments upon specimens which 

 have been anesthetized. After treatment with menthol or chloretone the 

 same power was exhibited. Examination of the tentacles shows the pres- 

 ence of large numbers of rounded concretions. These are packed tightly into 

 the endodermal cells. Their composition has not been determined. 



Experiments were made with a view to determining the specific gravity 

 of the organism. Solutions of magnesium sulphate in sea-water were pre- 

 pared, of various degrees of saturation. Inasmuch as no change in bulk, and 

 consequently none in density, was effected by temporary immersion in this 

 solution, it was concluded that the best way to determine the specific gravity 

 of the medusa would be to find a solution in which it would be just sus- 

 pended, without either sinking deeper or rising to the surface, and then 

 determining the specific gravity of this solution. This was the method sug- 

 gested by Dr. Mayer. It was found in this way that a solution, equal in 

 density to the jelly-fish, weighed 106.4 grams per 100 c. cm. That is to 

 say, the medusa, having the same density as the weighed solution, has a 

 specific gravity of 1.064, or 3-9 P er cent greater than that of sea-water. It 

 seems likely that the extra weight of the creature makes up for the defi- 

 ciency in the strength of the suctorial apparatus as compared with the corre- 

 sponding parts in the species from the Bahamas. 



IV. LIFE HISTORY. 



After finding the medusa of Cladoncnia in the moat of Fort Jefferson, 

 it was naturally a matter of interest to discover the other stages in the life- 

 history, if possible. So far as I have been able to determine, the hydroid 

 stage of the genus has never been reported from the open sea. The only 

 cases in which it has been seen have been those in which the creature has 

 made its appearance in captivity. The descriptions of these examples would 

 seem to indicate that they were entirely normal in all respects. It is of in- 

 terest, however, to find the hydroid growing in its natural environment. 



Many trips were made from the laboratory on Loggerhead Key to the 

 moat of the fort on Garden Key, 4 miles distant, and many hours were 

 consumed in a fruitless search for the polyp. A microscopic examination of 

 quantities of stones, sticks, and other debris from the bottom of the moat, 

 and of the plants and animals that make it their abode, failed to show any 

 sign of its existence. After some weeks had passed, however, the finding 

 of another hydroid on the alga which grows in abundance on the bottom of 

 the moat led to the discovery of the one I was more particularly anxious to 



