138 PLANT-ANIMALS [CH. 



period, the animal is ingesting solid food and there- 

 fore it is not impossible that the food-materials 

 obtained from this source may be shared alike by 

 the cells of the animal and the green, algal cells. 

 But, before we accept this view, we must enquire 

 into the conditions which obtain in the body of C. 

 roscoffensis at the time of infection, with the object 

 of ascertaining what sort of a "seed bed" for the 

 growth of the algal cells is provided by the body of 

 the animal. 



The first fact which is brought to light by an 

 enquiry of this nature is that the association between 

 green cells and animal does not begin with the en- 

 trance of the green cell or its colourless antecedent 

 into the body. Before the relationship reaches this 

 condition of intimacy, animal and free green alga have 

 struck up an acquaintance based on the identity of their 

 mode of phototropistic response. Under the stimulus 

 of unilateral light, they both move in the direction 

 of the light and both proceed to the upmost edge of 

 the sea-water pools or streams in which they occur. 

 But this is not all. The free-living alga settles 

 down from time to time in the mucilage which forms 

 a slimy coating over the body of C. roscofFensis and, 

 withdrawing its flagella, passes into the condition of 

 a resting cell (Fig. 22). Inasmuch as the skin of the 

 animal provides the capsules which enclose the 

 clutches of eggs, it follows that, not infrequently, one 



