82 C. CLIFFORD DOBELL. 
and varies considerably in size. About 16 w is an average 
length, but individuals are found of all sizes from about 75 u 
to 20. The breadth averages about 7-8 ». The anterior 
ends bears a flagellum (fl.), in length usually rather greater 
than the body. This flagellum arises from a depression at 
the anterior end ; the depression is a cytostome (est.), or cell- 
mouth, and leads into a longitudinal tube, the cytopharynx 
(cph.). This extends backwards for a variable length, and is 
somewhat spirally disposed. It can be seen, in carefully 
stained preparations, that the flagellum runs along the wall 
of the cytopharynx for a short distance, and takes its origin 
from a basal granule (bgr.), which is strongly coloured by 
Text-Fic. A. 
iron-hematoxylin. The base of the flagellum is in intimate 
relation with a large vacuole-like space (I.), the reservoir. 
Into this latter a minute contractile vacuole (cv.) rhythmically 
discharges its contents. Sometimes two small contractile 
vacuoles are present, instead of the usual one. - 
The reservoir itself does not pulsate, but from the fact that 
it is sometimes absent it may be inferred that it periodically 
collapses, driving out its contents. No communication with 
the cytostome can be made out, however. 
When the monad is at rest, either during division or when 
compressed by a coverslip, it can be seen that the contractile 
vacuole pulsates at the rate of once in about thirty seconds, 
It will be seen from the diagram that the reservoir, on its 
pharyngeal aspect, is related to the root of the flagellar 
