244 HYDROID POLYPES 



granules of deep red pigment. Their function is proved by 

 the following experiment. 



If a number of medusae are placed in a glass vessel of 

 water in a dark room, and a beam of light from a lantern is 

 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. 133). 



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 producing 

 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 apparently arise in the 

 endoderm of the stem of the hydroid colony, afterwards 

 migrating, while still small and immature, to their permanent 

 situation where they undergo their final development. 1 In 



1 This migration of the sexual cells renders the question of their 

 origin in many cases a very difficult one. In some Hydroids, at any 



