REPRODUCTION 



107 



ex.u. 



can.r. 



; can.c. 



Fig. 72. — A diagram of a vertical section 

 of the medusa of Obelia. The section is 

 supposed to pass on one side along a 

 radial canal, and on the other across 

 the endoderm lamella. In reality this 

 would not be possible, since the canals 

 are opposite one another. 



can.c. Circular canal ; can.r., radial canal ; en.L, 

 endoderm lamella ; ex.u., exumbrella surface ; g., 

 gonad ; j., jelly ; m., mouth ; mb., manubrium ; 

 n.r., nerve ring ; oes., oesophagus ; s.u., subum- 

 brella surface; st., stomach; stc., statocyst; 

 <^n., tentacle ; rm., velum. 



number and lie on the subumbrella below the radial canals. 



Each consists of a knob of ectoderm, into which passes a short 



branch from the radial canal. The germ-mother-cells originate 



in the ectoderm of the 



manubrium, migrate into the J «^- 



endoderm, and pass along the 



radial canals to the generative 



organs, where they migrate 



into the ectoderm again. 



When the ova or sperms are 



ripe, they are shed by the 



rupture of the ectoderm into 



the water, where fertilisation 



takes place. As in Hydra, 



segmentation leads to the 



formation of a hollow 



blastula. From this, by immi- 

 gration of cells at one spot, 



there is reached a stage with 



a solid mass of endoderm 



such as that found in Hydra. 



The animal at this stage is of a lengthened egg-shape and has 



a ciliated ectoderm, by which it swims freely for a while. It is 



known as a planula (Fig. 73). The 

 planula then settles down by its broader 

 end, an enteron is formed by a split 

 in the endoderm, tentacles and a 

 mouth form at the other end, and thus 

 there develops a polyp, from which a 

 colony arises by budding. When the 

 colony has reached a certain size there 

 appear, in the angles between the stem 



Fig. 73. — A, Planula larva; ^\ , , , i • v -u xu 



B, the young polyp into and the branches which bear the 

 which the planula grows hydranths, tubular outgrowths known 



as blastostyles, each enclosed m a vase 

 of perisarc known as a gonotheca. A blastostyle and its gonotheca 

 are together known as a gonangium. The blastostyle is probably 

 an incomplete zooid. On its sides are formed a number of buds 

 which develop into little medusae and escape through the opening 

 at the top of the gonotheca (Fig. 74). 



B 



