INTRODUCTION. 13 
Tue ConrractiLy 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 Amabz it follows the 
streaming of the endoplasm. In the testaceous forms 
(e. g. Arcella, Huglypha) 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, Actinophirys sol, one of the Heliozoa. 
e periphery of the body, bulg- 
Fic. 6.—Four stages in the contraction of the vacuole in Ameba 
proteus. xabout 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 
