4 SURFACE FAUNA OF THE GULF STREAM. 
nearly equal parts (Pl. I, Fig. 1). Seen from below (Pl. I, Fig. 3), the 
mantle is of a lighter bluish green color, with a light blue edge (the marginal 
glands), followed by a somewhat darker belt passing into the greenish color 
of the mantle. 
The tentacles (the closed prehensile polypites) are long, slender, of a bluish 
color, forming a double row round the outer edge of the float. The long- 
est only of these tentacles extend beyond the free edge of the mantle so as 
to be seen projecting beyond it, when the Velella is viewed from above. 
These tentacular polypites taper very gradually; they seem capable of but 
slight expansion and contraction, and are quite sluggish compared with the 
smaller, active, feeding and reproductive polypites. These are arranged in 
five or six rows between the rows of tentacular polypites and the large central 
polypite (Pl. 1, Fig. 5); the large, blue, prehensile, closed tentacular polypites 
are covered at the base (PI. VI, Fig. 17) by elliptical or circular patches of 
lasso-cells (Pl. VI, Fig. 16), which about halfway towards the extremity be- 
come more crowded, and unite so as to form a band of lasso-cells on each side 
of the polypite (Pl. VI, Figs. 14,15). In some young polypites the bands 
alone exist; while in others the elliptical patches alone are found (Pl. VI, 
Fig. 18). The smaller, the feeding and reproductive polypites are most ac- 
tive, and capable of great expansion and contraction. They are covered 
towards the upper extremity with elliptical knobs of lasso-cells (Pl. I. Figs. 
5,6, 7), the edge of the open extremity of the polypite forming ten to twelve 
indistinct lobes. At the base of the polypite there are, according to its size, 
from five to eight clusters of Medusze buds, in different stages of development 
(Pl. II, Figs. 1, 5, 7). While the large central polypite is the main feeding 
mouth, the smaller lateral ones also perform, to a limited extent, the func- 
tions of feeding polypites ; but, being all connected at their bases with the 
general vascular system, the fluids they take in enter at once into the general 
circulation. Both the central polypite, as well as the smaller lateral poly- 
pites, eject the digested substances which have gone through the general 
circulation. 
As has been shown by Weissmann, the circulation of the fluids in the 
coenosare in fixed Hydroids is kept up mainly by the muscular contraction 
of the walls, or by the action of the ciliz lining the cavities. A similar con- 
dition exists in the canals forming the vascular system of the float of Porpita 
and of Velelia, and in the polypites, where the fluids are rapidly propelled by 
the action of cilize lining the inner walls. 
en 
elgg 
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