320 OBELIA CHAP. 



their irritability. But no movement takes place without 

 such external stimulus, each stimulus giving rise infalli- 

 bly to one single contraction : the power possessed by 

 the entire animal of independently originating movement, 

 i.e., of supplying its own stimuli, is lost with the central 

 nervous system (compare p. 172). 



Another instance of morphological and physiological 

 differentiation is furnished by the marginal sense-organs 

 situated at the base of the tentacles (p. 315). 



The polype and medusa are respectively nutritive 

 and reproductive in function, the reproductive zooids 

 becoming detached and swimming off to found a new 

 colony elsewhere : the polypes are purely nutritive 

 zooids ; the medusae, although capable of feeding, 

 are specially distinguished as reproductive zooids. 

 Hanging at equal distances from the sub-umbrella, in 

 immediate relation with the radial canal, are four ovoid 

 gonads (Fig. 79, gon), each consisting of an outer layer 

 of ectoderm continuous with that of the sub-umbrella, 

 an inner layer of endoderm continuous with that of the 

 radial canal and enclosing a prolongation of the latter, 

 and of an intermediate mass of cells which have become 

 differentiated into ova or sperms. As each medusa 

 bears organs of one sex only (spermaries or ovaries, as 

 the case may be), the individual medusae are dioecious, 

 and not, like Hydra, monoecious. It will be noticed that 

 the gonad has the same general structure as an immature 

 zooid an out-pushing of the body-wall consisting of 

 ectoderm and endoderm, and containing a prolongation 

 of the enteric cavity. 



The medusae, when mature, become detached and 

 swim away from the hydroid colony. The sperms of the 

 males are shed into the water and carried to the ovaries 

 of the females, where they fertilise the ova, converting 

 them, as usual, into oosperms. 



