274 THE FOOD OF ANIMALS 



contention upon a comparison with monads and other flagellates 

 (see p. 267), and this doubtful organism consequently iigures in 

 many zoological works. 



Another problematic species is Volvox, a colonial freshwater 

 form in which the body is a hollow sphere with gelatinous walls, 

 in which are imbedded at regular intervals a large number of 

 individuals, each of which closely resembles the Berry Animalcule 

 in its motile stage. The colony is about the size of a small pin's 

 head, and progresses through the water, revolving as it does so, 

 by the united action of the numerous flagella. This organism is 

 very interesting on account of the symmetry of its body in all 

 directions, at any rate for a part of its life, and such a state of 

 things is exceedingly rare among living beings, and can only be 

 realized where light and other external agents act equally in all 

 directions. Only in a rotating aquatic form like Volvox is this 

 possible. 



The two examples which have just been given are instructive, 

 because they show the impossibility of drawing a sharp boundary 

 line between plants and animals. In the present state of our 

 knowledge part of the organic world will always be debatable 

 territory, claimed by both botanists and zoologists, and this is 

 not surprising when we reflect that plants and animals are almost 

 certainly descended from a common stock. 



