214 THE SEAS 



the plant in the association is not always clear but it is 

 probable that it obtains valuable food material from the 

 waste products of the animal it lives in. In the case of 

 the animal the advantage is more obvious ; in virtue of 

 its possession of the green colouring matter chlorophyll, 

 the plant, in the presence of sunlight, can manufacture 

 starch out of the water and the carbonic acid gas it holds 

 in solution, and this starch is utilized by the animal. The 

 majority of corals which always contain symbiotic algae 

 appear to gain a considerable, if not the greater, part of 

 their food in this manner. If other food is abundant the 

 algae are left undisturbed in the tissues of the coral but if 

 that is scarce then the coral appears to digest the plant:;, 

 a starved coral gradually digesting all its contained algae. 

 The most striking case of this type of symbiosis is furnished 

 by a small flatworm, Convolnta, only a few millimetres long, 

 which lives on the sandy shores of north Brittany and the 

 Channel Islands. It is bright in colour and occurs in large 

 colonies which form green patches on the yellow sand, 

 and is an animal of most regular habits, suddenly appearing 

 from beneath the sand immediately after the tide has left 

 it and disappearing just before the tide returns (Plate 82). 

 The green colour is due entirely to the presence in the 

 tissues of vast numbers of aigae. These are not present 

 in the egg but the animals become infected in a ver}' early 

 stdge in development, and if they are kept free from infection 

 by artificial means they fail to develop properly and soon 

 die. Although in early life it is able, like other flatworms, 

 to feed on smaller animals the Convoluta soon finds that it 

 is much simpler to depend entirely on the starch from the 

 green plants and, as a result of disuse, its digestive organs 

 degenerate so that it cannot, even if it wishes, feed like 

 a normal animal. This is the explanation of its regular 

 habits for in order to obtain the sunshine without which the 



