MUSEUM OF COMPARATIVE ZOOLOGY. 83 
The extended axis of the specimen captured was a foot and a half long. The 
size of the float, as compared with the diameter of the stem, is proportionally 
very large, as in Athorybia and Physalia, Its apex has a crimson color, and 
its apical walls are broken through by a circular opening. The longitudinal 
axis of the float swims vertical upon the surface of the water.* Cellular ap- 
pendages hang from the lower portion of the air-sac into the cavity of the float. 
The axis is slender, very contractile, and has a pale pink color, while that of 
R. filiformis is greenish in color. When the stem is retracted it forms a 
twisted snarl below the float, but at other times, when the Rhizophysa floats 
extended in the water, the tentacles and their side branches reach widely out- 
ward, and the polypites are turned at right angles to the stem. Just below 
the float the polypites are quite small, numerous, arise close together, and are 
destitute of tentacles, Their outer walls have a pale pink color, with more of 
an orange tinge than the axis and tentacles. The inner walls of the larger 
polypites bear characteristic “villi,” like those described by Huxley in Phy- 
salia, and likewise a prominent dark brown, almost black body, which closely 
resembles the “liver” of Velella. The “villi” on the inner walls of the polyp- 
ites of Rhizophysa are homologous with the well-known “tubes” which have 
been described in the liver of Velella. The polypites arise from all sides of 
the axis. 
Each of the larger polypites bears a single long flexible tentacle, which is 
destitute of tentacular knobs, but possesses filiform side branches, which are 
thickly set with large cells on one side, where the tentacular walls are en- 
larged. While many of the side branches are claret-colored, several are color- 
less. When the animal is captured, the tentacles must be almost torn from 
objects to which they fasten themselves, before it can be raised out of the water. 
The sexual organs resemble those of R. filiformis and R. gracilis F., and, like 
those of the former species, arise from the stem midway between two polypites. 
The close resemblance of the sexual clusters in Physalia and Rhizophysa has 
been pointed out elsewhere.t The side branches of the Rhizophysa tentacle are 
homologous with the reniform thickenings on the tentacles of Physalia.t 
* The longitudinal axis of the float of R. jiliformis lies horizontally on the sur- 
face of the water. 
7+ Bull. Mus. Comp. Zool., IX. 7. 
t The close likeness between the sexual organs in Physalia and Rhizophysa 
was pointed out in my description of R. gracilis in 1882. The comparison of the 
tentacular knobs in these two genera was made by Huxley (Oceanic Hydrozoa). 
Chun raises these two genera, Physalia and Rhizophysa, to the rank of an order, to 
which he gives the name “ Pneumatophoride ” (Pneumatophore), and which he 
regards as of equal rank with the Physophoride (Physophore) and the “ Calyco- 
phoride ” (Calycophore). In Rhizophysa gracilis F. we have a close approximation 
to Physalia in the structure of the body of the tentacle. The “sac,” loose folds on 
one side of this organ in Physalia, are likewise found very well developed in the 
closely allied Rhizophysa Eysenhardtii Geg. 
