96 PLANT-ANIMALS [CH. 



spherical shape and their chloroplasts become smaller 

 and rounder. Each reduced algal cell is now seen 

 to lie in a distinct, digestive vacuole containing a 

 pink fluid. Next, the pigment of the chloroplasts is 

 dissolved and, diffusing out of the cell, may impart a 

 brown colour to the vacuolar fluid. At this stage, 

 the chloroplasts are greenish ; later, they become 

 colourless. Finally, heaps of few or many, colourless, 

 curiously persistent granules are all that remain of 

 the algal cells. 



It is interesting to observe, in this connection, that 

 if animals are brought, after a prolonged sojourn in 

 darkness, into the light and supplied with fresh 

 sea-water, yellow-brown cells make their appearance 

 again in their bodies. As they grow and increase 

 in numbers, the animals also begin again to grow. 



So also in the case of C. roscoffensis, if the green 

 cells fail to make their appearance in the body, the 

 animals remain of microscopic size. If, on the other 

 hand, the green cells appear, increase and multiply 

 to form the characteristic green tissue, the animals 

 begin to grow rapidly. 



Thus in various ways it has been demonstrated 

 that C. roscoffensis and C. paradoxa depend for 

 their food on their coloured cells. Without them, 

 they fail to grow. When, by exposure to dark- 

 ness, the coloured cells are put out of photosyn- 

 thetic action, the animals become reduced in slice, 



