TYPES OF CCELENTERA 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 cavil y com- 

 municating with the exterior (g.p. 9 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- FIG. 74. Vertical section of Auretia.-- 

 matozoa pass from the Arter Claus - 



frills Of germinal eoi- "*> Mouth; st., stomach; r.c., radial canal; 

 . j?., reproductive organs; g-.f., gastric 



. A., reproductive organs; gj., gastric 



intO the SaCS, filaments; g-.p., sub-genital cavity; /., 



thpnrp into thp o-aQ- marginal tentacle; s., sense organ; 



the shaded part is mesogloea. 



trie 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 



