ECOLOGICAL 135 



from the sea, it can fall back on the sugar and the like which its 

 partners have made. In other words, it can absorb products of 

 their photosynthesis, and — why not, if need be? — even digest its 

 partners themselves. 



The same may be said in regard to most animals of low degree 

 that are green in colour. For while a few, like the green bell-animal- 

 cule {Vorticella viridis), have acquired chlorophyll of their own, 

 the great majority are green because of their partners. Thus there 

 are symbiotic Algae (Zoochlorellae) in the green freshwater sponges, 

 the green species of freshwater Hydra, some green sea-anemones, 

 many greenish corals, and some green worms. 



The symbiosis that has been most adequately studied (by Keeble 

 and Gamble) is that of a small green worm {Convoluta ro scoff ensis) , 

 common on the sandy shore at Roscoff, in Brittany. When the tide 

 goes down, the worm comes up; at the first splash of the returning 

 tide the Convoluta retreats again into the sand. This periodicity of 

 movement has become so engrained in the worm's constitution 

 that it persists for about ten days when the creature is transferred 

 to a tideless aquarium. It may even be exhibited in a test-tube! 

 But this is another story; let us keep to symbiosis. 



According to Keeble and Gamble the newly hatched Convolutas 

 are colourless, but in the course of a week they establish a symbiosis 

 with the Zoochlorellae. Unless this is accomplished they seem to 

 die, having apparently lost most of their power of fending for 

 themselves. After the establishment of the partnership there is a 

 period of true symbiosis, and the firm works well. The carbon 

 dioxide formed by the animal is used as food by the plant, which in 

 turn Hberates oxygen as long as it is exposed to the light. In the 

 course of time, however, there is a change in the relation, for the 

 Convoluta not only absorbs the carbon compounds that may 

 diffuse out from the Zoochlorellae, it begins to digest its partners as 

 such. Yet they seem to multiply rapidly enough to meet the demand! 



All the cases of probable symbiosis have not been so thoroughly 

 demonstrated as this case of Convoluta, but there is strong reason to 

 believe that many insects, such as cockroaches and larval death- 

 watches, have partner-yeasts in abundance in their food-canal, 

 and that these assist in a very important way in the fermentation 

 or digestion of the food, which is often of a dry-as-dust nature. 

 Some other animals have partner-moulds and many have beneficent 

 bacteria that certainly help in breaking down the food. The bacteria 

 find shelter and fodder, but it is difficult to prove that they are 

 restricted to these intestinal situations. Similarly there is strong 

 evidence that some luminescent animals, notably cuttlefishes, 

 owe their luminosity to colonies of bacteria, allied to the one that 

 illumines haddocks hung up to dry. But here again the proof 

 of a mutually beneficial partnership is very far as yet from being 



