STRUCTURE OF MEDUSA 1 87 



ridges of germinal epithelium, situated on the four patches 

 already mentioned, within sacs which are derived from and 

 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.p.^ Fig. 94). 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 con- 

 taining water and enveloping the genital sacs communicate 

 with the exterior only. 



The ova and spermatozoa pass from the frills of 

 germinal epithelium into the sacs, and thence into the 

 gastric 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. — The fertilised ovum divides completely, but not 

 quite equally, to form a blastula, with a very narrow slit -like cavity. 

 From the larger-celled hemisphere, single cells migrate into the cavity, 

 and fill it up with solid endoderm. The archenteron or primitive gut 

 cavity arises as a central cleft in this cell mass, and opens to the 

 exterior temporarily by the primitive mouth. During these processes 

 the embryo elongates, the outer cells become ciliated, and the mouth 

 closes. Thus the embryo becomes a free-swimming oval planula. 



After a short period of free lif% this planula settles down on a 

 stone or seaweed, attaching itself by the pole where the mouth formerly 

 opened. At a very early stage the mesogloea appears between the two 

 layers. At the free pole an ectodermic invagination next occurs, an 

 opening breaks through at its lower end, and thus a gullet lined with 

 ectoderm is formed, which hangs freely in the general cavity. During 



