14 BRITISH FRESHWATER RHIZOPODA. 



and the expulsion of the contents, step by step, under 

 a high power. Its reappearance he found to be always 

 somewhere near its point of disappearance. " While 

 still small it is carried along by the streaming proto- 

 plasm back to a position near the nucleus, where it 

 completes its development. The increasing weight of 

 the growing vacuole causes it to lag behind the stream- 

 ing granules and nucleus, until at its full growth it is 

 widely separated from the latter organ. The vacuole 

 may appear to move in the direction contrary to that 

 of the protoplasmic streaming, although in reality it is 

 quiescent; for while it remains in the field of the 

 microscope the main body of the animal moves well 

 out of it, until the vacuole is surrounded only by the 

 posterior end of the animal, which is reduced to a thin 

 layer of granules and a hyaline layer of ectoplasm 

 between the vacuole and the exterior. The granules 

 later move away, passing around the vacuole. until finally 

 there is only a thin layer of hyaline plasm between the 

 vesicle and the exterior. Shortly after this the vacuole 

 bursts and disappears, in most cases a distinct bulge 

 toward the outside preceding contraction. Contrac- 

 tion always begins on one side of the vacuole, and is 

 carried across it toward the outer edge."* 



The action of the contractile vesicle, doubtless, has 

 an important physiological meaning. The generally - 

 accepted view is that the periodical discharge is an 

 excretory function. The excess of water in the plasma- 

 body drains into the vacuole, and is thus got rid of. 

 In the more highly-organised Infusoria the water is 

 conveyed to the excretory vacuole by a system of ducts. 

 Such channels have not been seen in the endoplasm of 

 the Khizopoda ; but whatever the means employed by 

 Nature in their case may be, the effect is the same. 



INCEPTION OF FOOD. 



The food of the Rhizopoda is, in the main, chloro- 

 phyllous. It consists of diatoms, desmids, and spores 



* Calkins, op. cit., p. 88. 



