134 CCELENTERATA OR STINGING-ANIMALS. 



Reproductive System. — The sexes are separate. The repro- 

 ductive organs— ovaries or testes — consist of plaited ridges of 

 germinal epithelium, situated on the four patches already 

 mentioned, within sacs which are derived from and com- 

 municate with the floor of the gastric cavity. They are of a 

 bright red colour, and at first of a horse-shoe 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 within a sub-genital cavity communicating 

 with the exterior. The water which enters each of these 

 washes the outside of the genital sacs, and perhaps helps 

 in respiration. It must be clearly understood that the 

 genital sacs containing the plaited ridges of germinal epi- 

 thelium communicate with the gastric cavity only, while the 

 sub-genital cavities containing 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. 



Life- History of Aurelia. — The fertilised ovum divides 

 completely and equally. A hollow ball of cells — a blasto- 

 sphere or blastula — results. This undergoes invagination 

 into a two-layered gastrula. But the mouth or blastopore of 

 the gastrula closes, and a two-layered closed planula is 

 thus formed. This planula is externally ciliated, and may be 

 found in late summer swimming freely in the sea. But after 

 a short time the planula settles down on a stone or seaweed, 

 probably by its original blastoporal pole, acquhres a mouth at 

 the other pole, and proceeds to bud out tentacles — first four 

 corresponding to the angles of the mouth (perradials), and 

 then other four (interradials) between these, and then eight 

 intervening adradials. Internally, the cavity of this " Hydra- 

 tuba " or " scyphistoma " larva exhibits four inward-growing 

 interradial ridges or tseniolae. These correspond in position 

 to the gastric filaments of the adult, and are probably 

 comparable to the mesenteries of sea-anemones and other 

 Anthozoa. The Hydra-tuba is only about an eighth of an 



