100 LIFE IN THE SEA [CH. 



the worm almost as soon as it was born. It is quite 

 possible to rear the worm apart from the algae as a 

 colourless animal, and it is also possible to rear the 

 algae apart from the worm. When it is young the 

 worm feeds by ingesting diatoms, etc., but after a 

 time it ceases to take in any solid food. It obtains 

 its carbohydrate from the activity of the green cells, 

 which are able to synthesise starch from the carbonic 

 acid absorbed from solution in the sea-water, and 

 this starch is converted into sugar and is assimilated 

 by the other tissues of the worm. The alga obtains 

 its nitrogenous food from the products of excretion 

 of the worm, and for a time the association of the 

 plant and animal in one compound organism is of 

 advantage to both partners. But since the animal 

 has lost the power of feeding it becomes unable to 

 supply itself with proteid food, although it receives 

 abundance of carbohydrate food from the photo- 

 synthetic activity of the alga. Towards the end of 

 its life it therefore begins to digest the green cells in 

 order to obtain proteid food, and thus by cutting off 

 its supplies of carbohydrate it dies of starvation. 

 The earlier stage of the association of the two animals 

 is one of symbiosis, and it is of advantage to either 

 partner, but the later stage is one of parasitism on 

 the part of the worm. 



In this and some other cases the association 

 together of a plant and animal is an obligatory one, 



