190 
POPULAK SCIENCE EEYlEW. 
(fig. 10, c). It usually appears surrounded by a translucent 
ring (fig. 17 , n), which looks like a nucleated cell; but here 
again there exists doubt as to the true nature of the structure, 
partly, perhaps, arising from some want of exactness in the 
use of terms. Writers speak of a nucleolus within the nucleus, 
but Dr. Wallich affirms that this is merely the central portion 
of the nucleus, rendered more clear by the partial or total 
absence of the contained granules, which are crowded together 
at the circumference of the organ. The contractile vesicle 
(fig. 1, c) is a permanent and specialized vacuole of a remark- 
able character. Usually, there is but one present in each 
Amoeba; but in some circumstances there are more (figs. 2 & 8). 
This vesicle exhibits a rhythmical expansion and contraction, at 
intervals varying from half a minute to five minutes ; but in 
some instances the expanded, or diastolic, condition has been 
observed to continue above an hour. Mr. Carter seems to 
regard the vesicle as a cell with a proper wall. Dr. Wallich 
denies this interpretation, believing that the wall of the vesicle 
differs little, if at all, from the ectosarc. The vesicle evidently 
discharges some fluid at each systole, or contraction, but only 
does so at one point of the Amoeba (fig. 1, v) } where an anal 
outlet has been supposed to exist. When the vesicle contracts, 
it may wholly disappear, but always reappears at the same 
point. When the internal cavity has thus disappeared, its 
position may still be recognized by the aggregation of a number 
of minute villi, forming its outer surface, and which, when 
the vesicle is fully expanded, only appear at minute points. 
Some of these villi occasionally become distended, after a con- 
traction, forming a series of vacuoles around the central vesicle 
and which either burst into each other or into the primary 
cavity ; but sometimes one or two of these may become de- 
tached from the parent one, and float away into the sarcode. 
If these meet, they may burst into each other, as two soap 
bubbles would do, or they may return to the parent vesicle 
and burst into it ; but under no circumstances do they appear 
to contract until they have returned to the posterior, or villous, 
part of the body of the Amoeba (figs. 1 & 2, v). 
The food vacuoles (fig. 1, 3, 16, fv) are irregularly extem- 
porized cavities in the endosarc, within which particles of food 
are lodged, usually along with a drop of water. On coming 
in contact with any particle of organic substance on which the 
Amoeba is inclined to feed, and they obviously have some choice 
in this respect, the sarcode proceeds to surround it (fig. 3, a), 
so that it is soon inclosed (fig. 3 ,fv), along with some water, 
within the endosarc. Most observers agree in believing that 
any part of the body may thus incept the food, though some 
have believed in an oral aperture. Ho such aperture has been dis- 
