XXII GONADS 245 



allowed to pass through the water, the animals are all found 

 to crowd into the beam, thus being obviously sensitive to and 

 attracted by light. If however the ocelli are removed this 

 is no longer the case : the medusae do not make for the 

 beam of light, and are incapable of distinguishing light from 

 darkness. The ocelli are therefore organs of sight. 



In Zoothamnium we saw that the two forms of zooid were 

 respectively nutritive and reproductive in function, the re- 

 productive zooids becoming detached and swimming off to 

 found a new colony elsewhere (p. 136). 



This is also the case with Bougainvillea : 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 pro- 

 ducing ovaries, others spermaries only. Thus while Hydra 

 is monoecious, both male and female gonads occurring in the 

 same individual, Bougainvillea is dioecious, certain individuals 

 producing only male, others only female products. 



In some Hydroids it has been found that the sexual cells 

 from which the ova and sperms are developed do not originate 

 in the manubrium of a medusa, but arise in the first in- 

 stance from the ectoderm of the stem o the hydroid 

 colony, afterwards migrating, while still small and im- 

 mature, to their permanent situation where they undergo 

 their final development. In Bougainvillea, however, the 

 reproductive products are said to originate in the manubriunT). 



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 fertilize the ova, converting them, as 

 usual, into oosperms. 



