814 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 may be utilized by the animal. 



The most striking case of this type of symbiosis is fur- 

 nished by a small flatworm, Convoluta, only a few milli- 

 metres 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 algae. These are not 

 present in the egg but the animals become infected in a very 

 early stage in development, and if they are kept free from in- 

 fection by artificial means they fail to develop properly and 

 soon die. Although in early life it is able, like other flat- ■ 

 worms, 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 washes, 

 feed like a normal animal. This is the explanation of its 

 regular habits for in order to obtain the sunshine \vithout 

 which the plants cannot form their starch, it has to expose 

 itself to the full glare of the sun for as long as possible, 

 that is, be exposed on the sand for the whole time the tide 

 is out ; when the tide returns it has to burrow or it would 

 be washed away. 



But, o^ving probably to the fact that the Convoluta 



