34 PLANT-AXIMALS [CH. 



experiments on C. roscoffensis that a spell of illumi- 

 nation of brief duration favours one or other of 

 the series of processes which results in egg-laying, 

 that a longer or shorter spell of illumination is un- 

 favourable to this process, and that, when animals are 

 subjected to these unfavourable conditions, many of 

 them, though they are carrying eggs in an advanced 

 stage of development, remain sterile. 



With respect to the periodicity of egg-laying by 

 C. paradoxa it is not so easy to refer the periodic 

 character of this event to the influence of light. It 

 is noteworthy that other littoral, marine organisms, 

 (certain brown algae) living in almost identical habitats, 

 exhibit an identical periodicity with respect to their 

 reproductive processes. 



This, according to Williams (1898), is the case 

 with the brown sea-weed Dictyota dichotoma, and 

 subsequent observers have shown it to be true of 

 other marine algse. Not only does Dictyota liberate 

 successive crops of fertile eggs at fortnightly periods 

 but it sheds them at the same point in the tidal 

 period as that chosen by C. paradoxa for the dis- 

 charge of its egg-capsules. In either case, the eggs 

 are liberated some three to five tides after the 

 greatest springs. During the subsequent tides of 

 smaller amplitude, the zone which forms the habitat 

 of both sea-weed and plant-animal is continously sub- 

 merged. Hence we can scarcely escape the conclusion 



