HYDRA AND OBELIA. POLYPS AND MEDUSM 179 



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

 in the ectoderm of the manubrium, migrate into the 

 endoderm, and pass along the radial canals to the generative 

 organs, where they migrate into the ectoderm again. When 

 the ova or sperm 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 immigration of cells 

 at one spot, there is reached a stage with a solid mass of 

 endoderm such as 



that found in u. 



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. Anenteron FlG - "4- —A diagram of a vertical section of 

 the medusa of Obelia. The section is 



is formed by a split 

 in the endoderm, 

 and the planula 

 then settles down 

 by its broader end, 



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. 



forms tentacles and can - c -' ^ c j rcula i r ca if 1; c '" 1 - r -' ra u !a n cana ! ['' enM ' 



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

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

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

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

 ten., tentacle ; vm., velum. 



a mouth at the other 

 end, and develops 

 into a polyp, from 

 which by budding 



a colony arises. When the colony has reached a certain 

 size there appear, in the angles between the stem and the 

 branches which bear the hydranths, tubular outgrowths 

 known as blastostyles, each enclosed in a vase of perisarc 

 known as a gonotheca. A blastostyle and its gonotheca are 

 together known as a gonangium. The blastostyle is prob- 

 ably 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. 



It will be seen that Obelia, like Hydra, reproduces itself 

 both sexually and asexually. Sexual reproduction is carried 



