88 PLANT-ANIMALS [CH. 



of C. roscoffensis owes its origin to photosynthesis, we 

 demonstrate by the method which is used for a similar 

 purpose in the case of plants. The living animals 

 are kept in darkness and examined daily for starch. 

 After a time about seven or eight days in young 

 C. roscoffensis, about fourteen days in older animals 

 when, as indicated by the samples tested, starch has 

 disappeared having been converted into sugar and 

 used as food or in respiration the animals are 

 brought into the light and tested at intervals for 

 starch. As is the case with green plants treated 

 similarly, photosynthesis is resumed as soon as light 

 falls on the green cells, and within less than ten 

 minutes starch, which represents the reserve form of 

 the photosynthesised carbohydrate, makes its appear- 

 ance in the green cells. Moreover, the light which 

 is most efficient for photosynthesis in plants, that of 

 the red end of the spectrum, is also most efficient for 

 photosynthesis in the green cells of the plant-animal. 



It is not so easy to obtain rigid proof that the 

 yellow-brown cells of C. paradoxa are capable of 

 photosynthesis. Nevertheless, the indirect evidence 

 supports strongly the view that they do actually 

 function in this manner. 



In the first place, like similarly coloured alga3, 

 Avhich are known to manufacture their food photo- 

 synthetically, they possess a screening pigment and 

 also chlorophyll. 



