4 SUEFACE FAUNA OF THE GULF STIJEAM. 



nearly equal parts (PI. I, Fig. 1). Seen from below (PI. 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 (PI. I, Fig. 3) ; the large, blue, prehensile, closed tentacular polypites 

 are covered at the base (PI. VI, Fig. 17) by elliptical or circular patches of 

 lasso-cells (PI. VI, Fig. 16), which about half-way towards the extremity be- 

 come more crowded, and unite so as to form a band of lasso-cells on each side 

 of the polypite (PI. VI, Figs. 14, 15). In some young polypites the bands 

 alone exist; while in others the elliptical patches alone are found (PI. VI, 

 Fig. 18). The smaller, the feeding and reproductive polypites are most ac- 

 tive, and capable oi: great expansion and contraction. They are covered 

 towards the upper extremity with elliptical knobs of lasso-cells (PI. 11. Figs. 

 5, 6, 7), the edge of the open exti'emity 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 Medusse buds, in different stages of development 

 (PI. 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, tlie 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 

 coenosarc in fixed Hydroids is kept up mainly by the muscular contraction 

 of the walls, or by the action of the ciliae lining the cavities. A similar con- 

 dition exists in the canals forming the vascular system of the float of Porpita 

 and of Velella, and in the polypites, where the fluids are rapidly propelled by 

 the action of ciliae lining the inner walls. 



