SURFACE FAUNA OF THE GULF STREAM. 9 



Tlie young stages of Velella difler very materially in appearance from 

 the full-grown Velella. In the }oungest 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 (PI. 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 (PI. 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 (PI. VI, Fig. 2). The central polypite (PI. VI., Figs. 1, 2 cp.) 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 MedusJB he kept alive for a 

 few da^'s, 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, 

 having 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 Medusae he raised directly from Velella. 



The youngest Velella figured by Huxley (Oceanic Hydrozoa, PI. 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 



* Arcbiv f. Anat. Phys. u. Wis9. Med. 1870, p. 3(JG. 



