CH. X] THE CONDITIOXS OF LIFE IN THE SEA 227 



a certain degree of degeneration, except that the photo-synthetic 

 machinery and functions remain intact and unimpaired. 



What this means is that the green algal cells are able to 

 intercept the energy of sunlight, by virtue of the chlorophyll that 

 they contain ; so that they can build up starch from the carbon 

 dioxide and water of the medium in which they live. They 

 obtain their nitrogen from the waste nitrogenous products of the 

 metabolism of the body of the worm. Thus the mode of nutrition 

 of the green algal cells is holophytic, but they also betray a 

 tendency towards a saprophytic habit in that they can utilise the 

 nitrogen of such complex substances as urea, or uric acid. 



On the other hand the worm begins life as a typical animal ; 

 it captures and ingests organisms like diatoms, and this mode of 

 nutrition — a holozoic one — continues for about a week after birth. 

 If it does not become infected then with the alga it dies, probably 

 because it is unable to get rid of the nitrogenous waste products 

 of its own metabolism. The infection however always becomes 

 established in the state of nature, and then the excretory products 

 of the Convoluta are made use of as food by the algal cells, while 

 the starch elaborated by the latter is converted into sugar and 

 is used as food by the worm. Thus the association is a mutually 

 beneficial one for both organisms. 



We encounter similar cases of symbiosis among the corals 

 and coelenterates^ It is well known that much difficulty is 

 experienced in finding food materials in the digestive cavities of 

 tropical corals in sufficient quantity to serve for their proper 

 nutrition. Some time ago both Brandt and Hickson suggested 

 that the food-stuffs of these animals were supplied to them by the 

 Zoochlorellae which are often contained in their tissues. In the 

 paper by Miss Pratt, to which I refer, satisfactory evidence is 

 furnished that this is really the fact. The Alcyonaria of British 

 seas do indeed capture and ingest living prey, such as copepods ; 

 paralysing the latter by means of the poison threads of their 

 nematocysts, but these animals do not contain green algal cells. 

 In tropical Alcyonaria we find, however, that infection by Zoo- 

 chlorellae takes place, and that the increase of the infection is 



^ Miss Edith Pratt, "Digestive organs of the Alcyonaria," Quart. Journ. Micro. 

 Science, vol. xlix. p. 827, 1906. 



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