TYPES OF CCELENTERA — AURELIA AURITA. 141 



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 extend all round 

 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 

 communicating with the exterior (g.p., Fig. 60). The con- 

 tractions of the umbrella produce a rhythmic movement of 

 the water which enters the sub-genital sacs, and this constant 

 renewal of the water suggests some respiratory significance 

 for the sacs. It must be understood that the genital sacs 

 containing the plaited 

 ridges of germinal epith- 

 elium communicate with 

 the gastric cavity only, 

 while the sub - genital 

 cavities containing water 

 and enveloping the 

 genital sacs communicate 



with the exterior Only. Fig. 60. — Vertical section of. lurelia.— 



The ova and sperma- After Claus. 



t- n „ nn ,^„~,-, f_~™ 4-U^ f«;ilr. ni.. Mouth ; st., stomach ; r.c, radial canal ; 



tozoa pass from the frills ' A% r ; product i ve organ . s ' ; £ _ A gastr ; c 



Of germinal epithelium filaments; g-.jl., genital pocket; /., 



- . °, j ., marginal tentacle ; j., sense organ ; 



into the SaCS, and thence the shaded part is mesoglrea. 



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. 



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 

 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 life, 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 mesoglcea 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 



