14 SURFACE FAUNA OF THE GULF STREAM. 
(Pl. XI, Fig. 5), and are not arranged in sets of three, as they are represented 
by Kdlliker as characteristic of the Mediterranean species. The tubules com- 
mence about the outer edge of the white plate, and are most numerous towards 
its central part (Pl. XI, Fig. 2), where they come to the inner surface, 
and literally cover it with a matting of winding silvery threads (Pl. XI, 
Figs. 1, 4). Towards the centre of the float they become less numerous 
again (Pl. XI, Fig. 3), ending with six or seven tubules, which take their 
origin near the eight primary chambers, and extend over that part of the 
float. 
Porpita is not as easily upset as Velella; and the number of specimens 
thrown ashore by the winds is very small, as compared to the numbers of 
Velella stranded on the beaches after every storm. Porpita is capable of 
considerable control over its movements. Owing to the great size and power 
of its numerous long marginal tentacles, it can readily force itself back again 
into a normal attitude, if upset by the wind or waves. It can, by bringing 
its tentacles together over the disk (PI. X, Fig. 1), and throwing up the free 
edge of the mantle slowly in a given direction, then expanding the tentacles 
of one side far over on the opposite direction beyond the central part of the 
disk, it can thus readily change the centre of gravity, and tilt the disk back 
again into a normal attitude, should it from any cause have been set afloat 
with the tentacles uppermost. The larger outer marginal polypites are ar- 
ranged in three or four irregularly concentric rows, with two to three inner 
rows of smaller knobbed tentacles, in all stages of development (Pl. X, 
Figs. 2, 3, 10, 11). Inside of these are arranged, in from five to six similar 
rows, round the base of the large central polypite, the small, stout, flesh- 
colored feeding and reproductive polypites (Pl. X, Figs. 4, 5). These have a 
slightly rectangular head (Pl. VIH, Figs. 1-4), capable of considerable 
expansion, with four clusters of lasso-cells at the thick rounded angles 
of the terminal opening. At the base of these polypites are found Meduse 
buds in all stages of development (Pl. VII, Figs. 1-4; Pl. X, Fig. 4). At 
the time when in full reproductive power these clusters of Medusie completely 
fill the whole space between the small polypites, giving to the ring which they 
occupy on the lower surface of the float a dark yellowish tint from the color 
of the yellow cells found along the rudimentary proboscis of the Medusx 
buds, as well as along the chymiferous tubes (PI. VII, Figs. 1, 2, 4—11). 
The large marginal tentacles (PI. X, Fig. 3) are of a bluish tint, the ten- 
tacular knobs of a darker color. The internal cavity of the tentacles has a 
