804 ECOLOGY 



in the sunlight. Since the animal has no other source of food, it clearly is parasitic 

 on the alga. At first the animal uses the food (probably sugar) manufactured by 

 the green cells, but ultimately it destroys the cells themselves and brings on thereby 

 its own destruction. While it is possible that the alga utilizes substances in the 

 animal (in which event the relation is one of reciprocal parasitism), it is quite as likely 

 that the relation is to be regarded as a sort of destructive helotism, destructive be- 

 cause the enslaved organism is weakened and finally destroyed. In another flat- 

 worm, Convoluta paradoxa, the symbiotic alga is a unicellular brown species. 

 Here the parasitism of the animal seems less obligate than in C. rosco/ensis, since 

 it continues to use its mouth in taking food, even after the symbiotic algae are well 

 established in its body. However, the animal dies if it is kept in the dark until 

 the algae are destroyed. While the origin of such symbiosis is unknown, it may be 

 noted that in Noctiluca, one of the infusorians, symbiosis with green algae is faculta- 

 tive rather than obligate, thus suggesting a more primitive condition. Some of the 

 sea anemones contain algae which are believed to be of nutritive importance to the 

 animals, since the reactions of the latter to light resemble the reactions of algae 

 rather than those of such sea anemones as are without chlorophyll. It is likely 

 that the plants utilize the carbon dioxid given off by the animals, and that the animals 

 in turn utilize the oxygen given off by the plants. 



