SURFACE FAUNA OF THE GULF STREAM. 13 
As will be seen from the accompanying description, the other differences 
to be noted between the Florida and the Mediterranean species are: the 
numerous and close corrugations of the lower surface of the disk; its great 
roughness on the upper side; the comparatively greater length of the 
primary polypites ; the smaller size of the proliferous polypites. The shape 
of the former is very different from those of the Mediterranean species, ending 
in three to four large knobs, with three longitudinal rows of smaller ten- 
tacular knobs, —six in two of the rows, and from eight to nine in the 
central row (Pl. X, Figs. 3, 5, 7). 
As in the Mediterranean species, the disk is circular, slightly cup-shaped, 
convex above, made up of two disks, thickest near the outer edges, and 
united together by a series of circular walls entirely separated from one 
another (PI. XI, Figs. 7-13). The circular chambers thus formed open out- 
wardly by small elliptical openings giving air free access to them (Pl. XI, 
Figs. 7-9, 12,0). These chambers are filled with air (gas ?), giving to the 
Porpita its great buoyancy. The enclosed air (gas ?), shining through the 
float and the thin mantle which covers the disk, gives to its upper side a 
strong silvery lustre. The upper side of the disk is but slightly corrugated, 
with radial depressions, the stigmata being placed in the centre of low pro- 
jections, forming an irregular elevation in the lines of corrugations of the 
disk (Pl. XI, Fig. 7). There is a large central chamber, with an irregular 
ring of eight smaller ones (Pl. IX, Figs. 1-4), corresponding to the eight 
first-formed triangular chambers, placed round the central chamber. There 
does not seem to be beyond that first row any regular arrangement in the 
stigmata, or any order in their appearance or number. They vary greatly m 
position and in number in specimens of the same size. The stigmata are 
more numerous, as well as larger, near the edge of the disk, as has already 
been noticed by Kolliker. 
The ribbed structure of the lower side of the disk does not seem to have been 
observed. The lower floor of the radiating chambers extends into deep longi- 
tudinal corrugations (Pl. XI, Figs. 5, 8-11, 13, 14), forming immense pouches, 
as it were ; so that when seen endwise they present the appearance of high- 
pointed bags, with rounded tips, and deep spaces between them (PI. XI, 
Figs. 13, 14). A transverse (Pl. XI, Figs. 8, 9) and a longitudinal view 
(Pl. XI, Figs. 10, 11), will give a better idea of their peculiar structure than 
any lengthy description. To the under side of these pouches are attached the 
tubules (PI. XI, Figs. 1, 2, 3, 5, 8,9), they are irregularly placed in single rows 
