TYPES OF C@ELENTERA—AURELIA AURITA. 155 
communicate with the floor of the gastric cavity. They 
are of a reddish violet colour, and at first of a horseshoe 
shape, with the closed part of the curve directed outwards. 
Afterwards the ridges become circular, and surround the 
walls of the sacs in which they lie. But the sub-umbrellar 
surface is modified beneath each genital sac in such a way 
that the sac comes to lie in a sub-genital cavity com- 
municating with the exterior (g.4., Fig. 74). The con- 
tractions of the umbrella produce a rhythmic movement of 
the water which enters the sub-genital cavities, and this 
constant renewal of the water suggests some respiratory 
significance for the sacs. The genital sacs containing the 
plaited ridges of germinal 
epithelium communicate 
with the gastric cavity 
only, while the sub-genital 
cavities containing water 
and enveloping the geni- 
tal sacs communicate with 
the exterior only. 
The ova and = sper- Fic. 74.—Vertical section of Aurelia,— 
matozoa pass from the After Claus. 
frills of germinal epi- mM, Mau st., stomach ; Tbs radial canal; 
‘ ‘ +) Teproductive organs; g./., gastric 
thelium into the sacs, filaments; g.Z., sub-genital cavity; 2, 
and thence into the gas- "aging entices cs, sense organs 
tric cavity. They find 
exit by the mouth, but young embryos may be found 
swimming in the gastro-vascular canals, and also within the 
shelter of the long lips. 
Variations.—The jelly-fish often exhibits variations, i.e. 
inborn changes of germinal origin which result in the 
organism being different from the norm or average of its 
species. It is normally tetrapartite, but sexpartite, penta- 
partite, and, more rarely, tripartite forms occur; and the 
detailed variations are manifold. 
Life history of Aurelia.—The fertilised ovum divides completely, 
but not quite equally, to form a blastosphere, with a very narrow slit-like 
cavity. From the larger-celled hemisphere, single cells migrate into 
the cavity, and fill this up with a solid mass of endoderm. The 
archenteron arises as a central cleft in this cell mass, and opens 
to the exterior temporarily by the primitive mouth. During these 
