36 ANDREWS. 



their living substance, which seem to have become in some 

 way weakened or effete. 



[30] The so-called membrane of the contractile vacuole is 

 after all but an unstable, areal organization of the interal- 

 veolar substance, as a contractile pellicle, or membrane, 

 and besides its periodic effacements, is subject at all times 

 to the same flux of its substance as is the surrounding net- 

 work. Yet in so far as it is at times so modified in its 

 structure and reactions as to form, though but fleetingly, an 

 area of organized physiological reaction ; and to function at 

 such times as a contractile membrane ; it is a true organ of 

 the cell. 



It is a substance organ in the same sense as are all contrac- 

 tile pellicles. 



To relate in detail all the phenomena observed in the con- 

 tractile vacuole of Ajnoeba protetis alone, the rhythmical sub- 

 stance and structure changes, would fill a large essay, which it 

 is hoped to edit separately at some future date. 



In some Protoplasta, the number of contractile vacuoles is 

 not fixed, new ones arising capriciously in their midst. This is 

 true of some Ciliata also. 



I find that the contractile vacuole of Protoplasta is, as 

 a rule, but not always, discharged to the exterior of the 

 mass. At times, collapse takes place before the periphery 

 is reached. 



A strange intermittent contraction is found in the pellicle 

 which lines the pharyngeal and oesophageal cleft in the Vorti- 

 cellidse. By an annular constriction, the lower end of this 

 cleft which runs from the peristome down to the lower end of 

 the body is cut off, and then with its contents forms a sharp- 

 ended, pear-shaped sac full of water and of ingested food. 

 The contraction by which it was cut off passes on behind this 

 sac, pushing it further and further from the end of the now 

 much-shortened tube. By continuance of this same contraction 

 in a wave-like impulse, the sac is forced upwards through the 

 endosarc, along a gracefully curved course, until by pause of 

 the contraction it rests at some point. During this time, by 

 relaxation of the pellicular substance, it has yielded still more 



