DISCOPHORA. 81 
same relative position as the clapper does to an ordinary hand- 
bell, The distal end of the central polypite is furnished with a 
mouth, the lips of which are often prolonged into four longer or 
shorter lobes or processes. The mouth opens into a digestive 
sac, occupying the axis of the polypite ; and from the upper end 
of this proceed four radiating canals, which run in the substance 
of the disc to its margin, where they are united by a single cir- 
cular vessel, the whole system constituting the so-called “gastro- 
vascular” or “nectocalycine” canals. The margin of the bell 
is narrowed by a kind of shelf, which runs round the whole 
Zea 
— 
SE 
== 
b 
Fig. 26.—Naked-eyed Meduse. a Sarsia gemmiifera; b Modeeria formosa ; 
c Polyxenia Alderi (after Gosse). 
circumference, leaving a central aperture, and which is known 
as the ‘‘veil.” From the margin of the disc hang more or less 
numerous tentacles, which are hollow processes of the ectoderm 
and endoderm, and which communicate with the circular vessel 
of the canal-system. Also round the circumference of the 
swimming-bell are disposed certain “marginal bodies,” which 
are doubtless organs of sense. Some of these marginal bodies 
consist of little rounded sacs or “vesicles,” filled with a trans- 
parent fluid, and containing mineral particles, apparently of 
F 
