3IZ BOUGAINVILLEA chap. 



whole body of Hydra, is made over to the group of specially 

 modified ectodermal cells which constitute the central 

 nervous system. If a Hydra is cut into any number of 

 pieces each of them is able to perform the ordinary move- 

 ments of expansion and contraction, but if the nerve-ring 

 of a medusa is removed by cutting away the edge of the 

 umbrella, the rhythmical swimming movements stop dead : 

 the umbrella is in fact permanently paralysed. 



It is not, however, rendered incapable of movement, for 

 a sharp pinch, i.e. an external stimulus, causes a single con- 

 traction, showing that the muscles still retain their irritability. 

 But no movement takes place without such external stimu- 

 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 pigment spots or ocelli 

 (Fig. 76, c, oc) situated at the bases of the tentacles. They 

 consist of groups of ectoderm cells in which are deposited 

 granules of deep red pigment, and which serve as organs of 

 sight. 



The two forms of zooid — hydranth and medusa (p. 304) — 

 are respectively nutritive and reproductive in function, the 

 reproductive zooids becoming detached and swimming off 

 to found a new colony elsewhere : the hydranths are purely 

 nutritive zooids ; the medusae, although capable of feeding, 

 are specially distinguished as reproductive zooids. The 

 gonads are found in the walls of the manubrium, between 

 the ectoderm and endoderm, some medusae producing 

 ovaries, others spermaries only. Thus while Hydra is mon- 

 oecious (p. 302), Bougairivillea is, like the frog, dioecious.^ 



