150 THE BIOLOGY OF THE SEA-SHORE 



Similar relationships are not infrequent between plant 

 and animal. The plants are minute unicellular green or 

 brown cells known as Zoochlorellce and Zooxanthellce, and 

 they dwell within the tissues of some of the lower inverte- 

 brates, such as sponges, hydroids, sea-anemones and flat- 

 worms. The most striking partnership of this kind is that 

 between particular kinds of minute algae and the small 

 flat-worms Convoluta roscojfensis and C. paradoxa. The 

 appearance and occurrence of C. roscoffensis have been 

 described elsewhere (p. 239). The species C. paradoxa is 

 the less abundant of the two and occurs on seaweeds which 

 are epiphytic on larger weeds such as Ascophyllum and 

 Himanthalia. 



Both species of worm contain plant cells, which are green 

 in the case of C. roscoffensis and yellow in that of C. paradoxa. 

 The green cells are judged to belong to the Chlamydomona- 

 deae, and have been provisionally assigned to the genus 

 Carteria. They occur in large numbers just below the 

 surface of the body of the animal. It is sometimes stated 

 that the worm is dependent for its food on the products 

 of photo-synthesis on the part of the alga ; but this appears 

 to be not altogether correct, since Convoluta has not lost its 

 power of independent nutrition. Apparently, one function 

 of the plant cells, at a particular period, at least, is to serve 

 as an excretory system to the worm, by receiving (it is 

 thought) nitrogenous decomposition products from its 

 amoeboid cells. These wandering amoeboid cells act as a 

 digestive system to the worm by ingesting and circulating 

 food particles. The worm contains no green cells when 

 born, but these obtain entry from the sea-water later on. 

 When once they have entered the tissues of the worm 

 the plant cells become modified and lose their nucleus so 

 that they are thereafter incapable of leading a free existence. 

 At first, as we have already stated, they serve as an excretory 

 system to the worm, but later on, apparently, the worm 

 turns on its ally and assimilates it into its own tissues. 

 The relationship between plant and worm thus varies 

 with age. It is remarkable that while Convoluta feeds 



