iv] GREEN CELLS OF CONVOLUTA 103 



doned their free and independent modes of life and 

 have taken up their abodes in the tissues of animals, 

 yet they do not constitute a final proof of the truth 

 of this hypothesis. Indeed, the problems presented 

 by the chlorophyllous cells of animals are too 

 numerous and important to be dismissed by means 

 of a loosely-drawn inference of this sort. To the 

 possession of chlorophyll the plant owes its powers 

 of photosynthetic manufacture ; and to the absence of 

 this pigment from the cells of animals is due the 

 dependence of the animal world on the world of 

 plants for food supplies. Yet, low down in the 

 animal kingdom, organisms exist which, though un- 

 doubtedly possessed of distinct animal characteristics, 

 contain chlorophyll and use it for the manufacture 

 of carbohydrate food. Thus, species of Euglena 

 (e.g. E. viridis), which stand near the parting of the 

 ways which lead, the one to the animal kingdom, 

 the other to the vegetable kingdom, contain chloro- 

 phyll and use it for photosynthetic purposes. Now 

 Euglena viridis is undoubtedly an animal. The single 

 cell or protoplast of which it consists is provided 

 with a gullet, into which solid particles may pass 

 and thus be ingested by the animal. The membrane 

 which encloses the organism is not composed of 

 cellulose the cell-wall substance of typically vege- 

 table organisms ; and in yet other ways Euglena 

 gives evidence of its "animal" nature. 



