THE INFUSORIANS. 31 
of Protoplasta. Other Gregarine are very minute, and are 
parasitic in insects (Fig. 19), etc., and vary greatly in form, 
some being apparently segmented, while in a few forms the 
body ends anteriorly in a sort of beak armed with recurved 
horny spines. We are now prepared to adopt the following 
definition of the class : 
Cuass III.—GREGARINIDA. 
Ameba-like Protozoa, more or less elongated, with a determinate cell- 
wall, with a subcuticular system of muscular fibrille, with a nucleus, but na 
contractile le; reproducing by encysting and subdivision of the cen- 
tral mass of the body, producing shelly psorosperms, from which escape the 
moner-like young, which undergo a metamorphosis into the usually worm- 
shaped, parasitic adult (Gregarina). 
Cuiass IV.—INFUSORIA. 
These organisms can best be understood by studying rep- 
resentatives of the three orders forming the class,* 
Order 1. Flagellata (Monads).—A familiar example of 
monads, Oikomonas termo Clark, has been studied by H. 
J. Clark. His description will suit our purpose of indi- 
cating the form and habits of a typical flagellate animaleule. 
It somewhat resembles our figure of Uvella in its general 
shape, being pear-shaped, faint olive in color, and provided 
with a vibratile locomotive lash or flagellum. In swimming, 
the monad stretches out the flagellum, which vibrates with 
an undulating, whirling motion, and produces a peculiar 
graceful rolling motion. When the monad is fixed the fla- 
gellum is used to convey food to the mouth, which lies be- 
tween the base of the flagellum and beak, or “‘lip.’’ The 
food is thrown by a sudden jerk, and with precision, directly 
against the mouth. ‘‘ If acceptable for food, the flagellum 
presses its base down upon the morsel, and at the same time 
the lip is thrown back so as to disclose the mouth, and then 
bent over the particle as it sinks into the latter. When the 
lip has obtained a fair hold upon the food, the flagellum 
withdraws from its incumbent position and returns to its 
former rigid, watchful condition. The process of degluti- 
* Kent’s Manual of the Infusoria, London, 1880; Stokes’ Microscopy ° 
for Beginners, 1887. 
