n NUTRITION 253 



oxygen in sunlight : as in Sphaerella the carbon dioxide 

 in solution in the water is decomposed in the presence 

 of chlorophyll, its oxygen evolved, and its carbon com- 

 bined with the elements of water and used in nutrition. 

 For a long time Euglena was thought to be nourished 

 entirely in this way, but there is reason for thinking that 

 this is not the case. 



When the anterior end of a Euglena is very highly 

 magnified, it is found to have the form shown in Fig. 69, 

 A, B. It is produced into a blunt snout-like extremity, 

 at the base of which is a conical depression (gul) : 

 just the sort of depression one could make in a clay 

 model of Euglena by thrusting one's finger or the end 

 of a pencil into the clay. The bottom of this tube opens 

 into a space, the reservoir (r), from one side of which 

 the flagellum arises by a bifurcated base, and by its 

 continual movement gives rise to a sort of whirlpool 

 in the neighbourhood. By the current thus produced, 

 minute solid food-particles are said to be swept down 

 the tube and forced into the soft internal protoplasm, 

 where they doubtless become digested in the same way 

 as the substances ingested by an Amoeba. That solid 

 particles are so ingested by many unicellular organisms 

 (see e.g., pp. 266 and 272) has been proved by diffusing 

 finely powdered carmine in the water, when the coloured 

 particles were seen to be swallowed in the way described. 



The depression in question serves therefore as a gullet, 

 and its external aperture or margin (m) as a mouth. 

 Euglena, like Amoeba, may take in solid food, but 

 instead of ingesting it at any part of the body, 

 it can do so only at one particular point where there is a 

 special ingestive aperture or mouth. This is clearly a 

 case of specialisation or differentiation of structure : in 

 virtue of the possession of a mouth and gullet, Euglena 

 is more highly organised than Amoeba. 



