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 water 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 per 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; LIEBE HISTORY. 
After finding the medusa of Cladonema 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 débris 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 
