320 OBELIA CHAP. 



lus, each stimulus giving rise infallibly to one single con- 

 traction : 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 bases 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 else- 

 where : the polypes are purely nutritive zooids ; the me- 

 dusae, 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 endo- 

 derm, 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. 



The oosperm undergoes segmentation, forming a poly- 



