NUTRITION 295 



SYMBIOSIS 



In certain unicellular animals, such as Euglena, we find chlorophyll grains, as 

 if in a plant cell. Some species of multicellular animals, also, such as Hydra 

 viridis, and the planarian worm, Convoluta Roscoffensis, take in algae and carry 

 on a partnership in nutrition, as it were. Keeble (1910) shows that this Convoluta, 

 after it has taken up the green algae, is able to live and grow in sea water which 

 contains no solid particles, and only traces of organic matter of any kind. The 

 algse are able to decompose carbon dioxide and form carbohydrate, as under 

 normal conditions. It appears that the starch, thus formed, is changed into fatty 

 substances by the vegetable cells and passed on to the animal cells in this form. 

 Starch itself cannot be attacked by the cells of the animal. Rows of fatty particles 

 are seen apparently passing from the green cells to the neighbouring animal cells. 

 It is, of course, possible that sugar also may pass, as such, to the latter cells. 



Without the green cells, Convoluta Roscoffensis fails to grow, and the same 

 thing happens in darkness. At a certain stage of its existence, the organism 

 ceases to take in solid food and depends entirely on its vegetable partners. 



Keeble has shown further (1910, p. 123) that the infecting algae are capable of 

 independent existence. . 



An interesting question is why the infecting algae grow so rapidly as they do 

 inside the animal organism. It is obvious that they must obtain nitrogen and it 

 is very significant that Convoluta Roscoffensis and C. paradoxa, which also 

 contains symbiotic algae, are peculiar amongst the Turbellarian worms in possessing 

 no excretory system for the waste products of their nitrogenous metabolism. The 

 conclusion is clear ; these waste products are utilised by the algae. In fact, it is 

 actually found that these particular algee grow better when supplied with their 

 nitrogen as uric acid than as nitrate. There is, moreover, evidence that, not only 

 do the cells of the algae supply the animal cells with fat and carbohydrate, but also 

 with nitrogenous food, which they are able to hand on after having converted that 

 obtained from the sea water into a form with which animal cells can deal. 



The reader is recommended to consult the fascinating little manual by Keeble 

 (1910) for further information. 



SUMMARY 



The use of food in the growing organism is to supply material for construction 

 of body substance, to replace that lost in wear and tear, and to give energy for 

 the performance of muscular movements as well as for the bringing about of 

 endothermic reactions. In the adult, of course, the necessity of a supply for 

 growth is absent. 



The amount required for the replacement of wear and tear, or maintenance, 

 is small. 



In growth, it is obvious that all the chemical elements which are constituents 

 of the organism must be supplied in some form or other, but, while very simple 

 chemical compounds suffice for the lower organisms, the capability of dealing 

 with such is, to a large extent, lost by the animal, even at a comparatively early 

 stage of the protozoa. These require food in the form of complex organic 

 compounds already prepared by other animals or plants. As carbon, nothing 

 less complex than glucose ; as nitrogen, nothing less complex than amino-acids 

 suffice for these animal organisms. 



In addition to the known organic compounds, the presence of traces of some 

 substances, whose constitution is as yet unknown, is necessary, not only for 

 growth, but also for maintenance. It appears, however, that there are some 

 of these which are absolutely indispensable for growth, but unnecessary for 

 maintenance. These "accessory factors" do not act by forming part of the 

 constitution of definite chemical compounds such as the proteins of the protoplasm, 

 but as " hormones " or catalysts ; although the possibility of their forming some 

 essential part of the cell mechanism, such as the surface membrane, has not yet 

 been definitely excluded. 



