20 ANIMAL PHYSIOLOGY. 
tion of the struggle for existence and the survival of the fittest. 
If the cells of an organism are mightier than the bacteria, the 
latter are overwhelmed; but if the bacteria are too great in 
numbers or more vigorous, the cells must yield; the battle may 
waver—now dangerous disease, now improvement—but in the 
end the strongest in this, as in other instances, prevail. 
UNICELLULAR ANIMALS WITH DIFFERENTIATION OF 
STRUCTURE. 
THE BELL-ANIMALCULE (Vorticella). 
Ameeba is an example of a one-celled animal with little per- 
ceptible differentiation of structure or corresponding division 
of physiological labor. This is not, however, the case with all 
unicellular animals, and we proceed to study one of these with 
considerable development of both. The Bell-animalcule is 
found in both fresh and salt water, either single or in groups. 
It is anchored to some object by a rope-like stalk of clear pro- 
toplasm, that has a spiral appearance when contracted; and 
which, with a certain degree of regularity, shortens and length- 
ens alternately, suggesting that more definite movement (con- 
traction) of the form of protoplasm known as muscle, to be 
studied later. 
The body of the creature is bell-shaped, hence its name; the 
bell being provided with a thick everted lip (peristome), covered 
with bristle-like extensions of the protoplasm (cilia), which are 
in almost constant rhythmical motion. Covering the mouth of 
the bell is a lid, attached by a hinge of protoplasm to the body, 
which may be raised or lowered. A wide, funnel-like depres- 
sion (wsophagus) leads into the softer substance within which 
it ends blindly. The outer part of the animal (cuticula) is 
denser and more transparent than any other part of the whole 
creature; next to this is a portion more granular and of inter- 
mediate transparency between the external and innermost 
portions (cortical layer). Below the disk is a space (contractile 
vesicle) filled with a thin, clear fluid, which may be seen to 
enlarge slowly and then to collapse suddenly. When the Vorti- 
cella is feeding, these vesicles may contain food-particles, and 
in the former, apparently, digestion goes on. Such food vacu- 
oles (vesicles) may circulate up one side of the body of the ani- 
mal and down the other. Their exact significance is not known, 
but it would appear as if digestion went on within them; and 
