286 THE PROTOZOA 



upon proteids, so that in this secondary vacuole the real disinte- 

 gration of the food substance takes place. 



According to Greenwood's observations, therefore, it appears that 

 the original vacuole does not become the digestive vacuole, but that 

 the food particles first become free in the protoplasm, to be re-collected 

 and digested in an acid-holding gastric vacuole. 



There are no observations to indicate the changes which take place 

 in the disintegrated food particles from the time they leave the gas- 

 tric vacuole until the absorption, by intussusception, of the nutritive 

 parts contained in them. Most observers, however, are agreed that 

 there must be a sort of chyme formed which mixes with the proto- 

 plasmic fluids and is absorbed by them. 



Closely connected with the metabolism of the protozob'n cell, but 

 as yet of unknown origin, are the so-called "excretory granules" 

 (Assimilationskorperchen of Rhumbler, Excretkorner of Biitschli, 

 Schewiakoff, corpuscules birefringents of Maupas, etc.). The wide 

 distribution of these granules, or crystals, their formation and disap- 

 pearance under varying conditions of the organisms, make it probable 

 that they play an important part in the physiology of the Protozoa. 

 Whether, however, they represent a final stage in the processes of 

 digestion, or represent the products of katabolic metabolism, has not 

 been satisfactorily made out. 



The granules in question, with or without a definite crystalline 

 form, have been described in almost every class of Protozoa. In 

 shelled and in naked fresh-water rhizopods (Auerbach, '55; Carter, 

 '64; Lankester/79; F. E. Schultze,'/5 ; Maupas, '83; Schewiakoff, '88), 

 in Heliozoa(Hertwig& Lesser, '79; Maupas,'83), in Flagellidia (But- 

 schli, '78, '83), and in Infusoria (Maupas, '83; Stein, '59; Wrzesniowski, 

 '79; Rhumbler, '88; Schewiakoff '94), they occur in varying numbers 

 and positions. In the Ciliata, where they have been most thoroughly 

 studied, they may lie well distributed about the endoplasm (Fig. 148), 

 or may be concentrated at the two ends of the animal in the 

 vicinity of the contractile vacuoles (Paramcecium ; cf. Schewiakoff, 

 '94). When distributed about the body, they lie in vacuoles which 

 move freely with the protoplasmic flow. They vary considerably in 

 form (C), but, as a rule, have a crystalline appearance, while 

 the larger granules are striated by radial or parallel lines, indicating, 

 Schewiakoff believes, the coalescence of needle-form crystals. They 

 vary in size according to the degree of aggregation, but measure, on 

 the average, from 0.003 to 0.014 mm. in length (Schewiakoff). 



Numerous suggestions as to the significance of these crystals, and a 

 few valuable experiments to determine their chemical composition, 

 have been made. From analogy with other animals it was early sup- 

 posed that they represent concretions which correspond to uric acid 



