SURFACE FAUNA OF THE GULF STREAM. 9 
The young stages of Velella differ very materially in appearance from 
the full-grown Velella. In the youngest stage I have had the opportun- 
ity to examine, the sail forms a flat elliptical arch (PI. VI, Fig. 8), ex- 
tending well beyond the extremities of the conical float, the mantle form- 
ing an umbrella-shaped projection, below which extend, when expanded, 
the eight marginal tentacles and the large central polypite. The rudimentary 
small feeding polypites are about as numerous as the tentacles, and form an 
inner ring at their base, much as in the older stages figured here (Pl. VI, Figs. 
2,3). The vascular system extends in straight vessels radiating from the 
float through the mantle to the outer edge. In a somewhat older stage, seen 
from the lower side (Pl. VI, Fig. 3), the marginal tentacles and feeding 
polypites are more numerous, the vessels of the liver system more clearly 
defined, the sail has become somewhat conical, and the whole float and 
mantle somewhat flattened. In a still more advanced stage (PI. VI, Fig. 1), 
the sail has become more conical, the float greatly flattened, and the canals 
of the liver system clearly defined, as is well seen in a view from the lower 
side (Pl. VI, Fig. 2). The central. polypite (Pl. VI., Figs. 1, 2 ep.) at this 
stage is most prominent, capable of great expansion; the feeding polypites 
have greatly increased in number as well as size; there are as yet no signs 
of the reproductive polypites. 
Stuart* has given a detailed development of the Medusa of Velella, and 
traced it directly to a free Medusa with four chymiferous tubes with large 
masses of yellow cells along the tubes. These Meduse he kept alive for a 
few days, but could trace no further stages of development. The oldest 
Medusa he observed showed as yet no sign of any tentacular appendages. 
Stuart, however, seemed satisfied that the Chrysomitra of Gegenbauer, 
haying sixteen chymiferous tubes and two distinct tentacles with tentacular 
knobs at the base of the tubes, was really a more advanced stage of the 
Medusa of Velella, though he did not observe the intermediate stages of 
development between it and the Meduszx he raised directly from Velella. 
The youngest Velella figured by Huxley (Oceanic Hydrozoa, Pl. XI, Fig. 9) 
measured about one tenth of an inch in length. He noticed that the pneu- 
matocyst did not extend into the crest, in which he also saw the rudimentary 
canals extending from the limb to the crest. On examining the pneumato- 
cyst he found a single central vesicle with the first trace of a concentric line, 
the rudiment of the first concentric chamber. This structure agrees well 
* Archiv f. Anat. Phys. u. Wiss. Med. 1870, p. 366. 
