28 INVERTEBRATE ANIMALS. 
fore acts as an organof touch. The power of active locomotion 
is enjoyed by most of Protozoa; but in some cases this is very 
limited, and in other cases the animal is permanently fixed (as 
in the sponges). The apparatus of locomotion in the Protozoa 
is of a varied nature. In many cases, especially in the higher 
forms, movements are effected by means of little hair-like pro- 
cesses, which are called “cilia” (Lat. c¢/éwm, an eyelash), and 
which have the power of vibrating or lashing to and fro with great 
rapidity. In other cases the cilia are accompanied or replaced 
by one or more long whip-like bristles, which act in the same 
fashion, and are known as “ flagella.” Amongst the lower Pro- 
Zozoa the most characteristic organs of locomotion are the so- 
called ‘“‘pseudopodia” (Gr. pseudos, falsity ; fodes, feet). These 
consist of variously-shaped filaments, threads, or finger-like pro- 
cesses of sarcode, which the animal can thrust out from any or 
every part of its body. They are not, however, definite and per- 
manent organs like the cilia, for they can be produced at will, 
and when they are again withdrawn they simply melt into the 
sarcode of the body, and leave no traces of their existence. 
As regards the classification of the Protozoa, a rough and use- 
ful division is into mouth-bearing or “stomatode” Protozoa, in 
which there is a distinct mouth; and mouthless or “astomatous ” 
Protozoa, in which there is no mouth. It issomewhat doubtful, 
however, if the mouth-bearing forms (namely, the Jz/fusoria) 
can properly be kept in the Protozoa, so that this arrangement 
is not a very good one. More scientifically, the Protozoa are 
divided into three great divisions or “classes,” known by the 
names Gregarinide, Rhizopoda, and Infusoria, all of which 
require special examination. 
CLASS I. GREGARINIDZ.—The Gregarinide may be defined 
as parasitic Protozoa which have no mouth, and have no power 
of giving out pseudopodia. They are usually looked upon as 
forming the lowest class of the Profozoa ; but in all probability 
much of their degraded character, as we shall see in other 
cases, is due to the fact that they are internal parasites, and 
are therefore not dependent on their own exertions for food. 
They vary in size from less than the head of a small pin up to 
nearly half an inch in length, when they look something like 
small worms; and they are found inhabiting the intestines of 
various animals, especially the cockroach and the earth-worm. 
In anatomical structure a Gregarina usually presents the 
