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 Athonjbia 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 Ehizophysa 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 Ehizophysa 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 nuist 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 polj'pites. 

 The close resemblance of the sexual clusters in Physalia and Ehizophysa has 

 bee"!! pointed out elsewhere. f The side branches of the Rhizophysa tentacle are 

 homologous with the reniform thickenings on the tentacles of Physalia.X 



* The longitudinal axis of the float of R. filiformis lies horizontally on the sur- 

 face of the water. 



t Bull. Mas. Comp. Zm31., IX. 7. 



X The close likeness between the sexual organs in Physalia and Ehizophysa 

 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 Ehizophysa, to the rank of an order, to 

 which he gives the name " Pneumatophoridae " (Pneumatophorae), and which he 

 regards as of equal rank with the Piiysophoridae (Physophorffi) and the " Calyco- 

 phoridae " (Calycophora^). In Ehizophysa 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 Ehizophysa Eysenhardtii Geg. 



