8 SUEFACE FAUNA OF THE GULF STEEAM. 



The mantle covering the float extends, as is well known, not only over the 

 horizontal surface of the float, but also over the sail. It projects beyond that, 

 forming a sort of flap (PI. V, Fig. 4), much as the mantle projects beyond the 

 horizontal part of the float. From the two extremities of the float, at the base 

 of the keel or sail, there runs along the free edge a large tube of the vascular 

 system (described by Kolliker and Vogt), from which branch the dendritic 

 processes forming the triangular patches (PL V., Figs. 4, 5) of the free edge of 

 the mantle of the keel. This free sail mantle is of a light claret color, with a 

 blue edge, and with bluish branching tubes forming the ramifications of the 

 vascular system. These tubes anastomose again at the outer edge, forming an 

 irregular marginal canal. There are no glands to the free edge of the keel 

 mantle (PI. V, Fig. 5), like those found on the free edge of the horizontal raaa- 

 tle of the float. The yellow cells of the sail mantle are packed principally in 

 patches at the extremities of short tubes opening into the main canal, fringing 

 the keel at the base of the free sail mantle (PI. IV, Fig. 5, PI. V, Figs. 2, 10). 

 The dendritic tubes are a series of flattened elliptical pouches, opening into 

 one another, and joined together by friU-like folds of the main tubes (PI. V, 

 Fig. 7). The two surfaces of the mantle join at the edge of the float, so that 

 the part of the mantle which covers the sail and extends to the outer edge 

 of the float, unites there with that part of the mantle which protects the 

 inner side of the float, and to which the appendages of the lower surface are 

 attached. These two surfaces, thus soldered together, extend some distance 

 beyond the float, forming the free edge of the mantle of the Velella. The 

 mantle itself is slightly contractile, and whenever the Velella is thrown 

 over into any unnatural attitude, or forced on its side, it makes violent 

 attempts by the movement of its prehensile tentacles, aided by movements 

 of the free margin of the mantle, to recover its normal attitude. 



Rhizophysa and other Siphonophores are capable of sinking below the 

 surface and swimming back to the surface, but neither Velella nor Porpita 

 appear capable of such movements, a very young Physalia, collected at the 

 Tortugas, intermediate between the stages figured by Huxley (Oceanic 

 Hydrozoa, PI. X, Figs. 1, 2), was found to swim at various levels in the jar 

 in which it was kept. 



All the Velella? floats I have examined are left-handed, that is, the sail 

 runs northwest to southeast, the longitudinal axis of the float being placed 

 north and south. I have counted over twenty-five hundred dead floats, 

 thrown on the beaches at the Tortugas, in all of which the position of the 

 float was as stated above. 



