INTRODUCTION'. 



THE CONTRACTILE VACUOLE. 



The freshwater Rhizopoda, with but few exceptions, 

 exhibit in some part of the plasma at least one con- 

 tractile vesicle, of which the functions are as yet but 

 imperfectly understood. In the Amwltse it follows the 

 streaming of the endoplasm. In the testaceous forms 

 (e. g. Arcella, Euglyplia} the number of vacuoles varies ; 

 they occupy a more definite place in the protoplasm, 

 and are, as a rule, 'of smaller size. 



The action of the vesicle is best seen in the familiar 

 sun-animalcule, Actinoplirys sol, one of the Heliozoa. 

 It is there situated on the periphery of the body, bulg- 



m 



FIG. 6. Four stages in the contraction of the vacuole in Amoeba 

 proteus. x about 400. (After Calkins.) 



ing outwards conspicuously during diastole, and, if 

 note be taken of the intervals of discharge, the time 

 will be found to approach sixty seconds. The fre- 

 quency of the pulsations, however, is not the same in 

 all species. The period between diastole and systole 

 is longest in the testaceous forms. 



There is considered to be an indefinable relation 

 actual connection has not been proved between the 

 contractile vesicle and the nucleus. It is found that, 

 as the vesicle gets filled it falls towards the posterior 

 region by reason of its greater weight as compared 

 with the enclosing endoplasm, and that finally reaching 

 the ectoplasm its contents are discharged outwards. 

 This, however, is not easily demonstrated. Calkins 

 traced the formation and contraction of the vacuole 



