8 SURFACE FAUNA OF THE GULF STREAM. 
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 (Pl. 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 man- 
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, Pl. V, Figs. 2, 10). 
The dendritic tubes are a series of flattened elliptical pouches, opening into 
one another, and joined together by frill-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, Pl. X, Figs. 1, 2), was found to swim at various levels in the jar 
in which it was kept. 
All the Velellw 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. 
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