INVERTEBRATE ANIMALS. 
SUB-KINGDOM I. PROTOZOA. 
CHAPTER I. 
1. GENERAL CHARACTERS OF THE PROTOZOA. 2. CLASSI- 
FICATION. 3. GREGARINIDA. 
Tue sub-kingdom Protozoa (Gr. protos, first; and zoo, an 
animal), as the name implies, is the lowest division of the 
animal kingdom, and its limits are therefore necessarily not yet 
strictly defined. The Protozoa comprise an enormous number 
of animals, almost all of which are so small as to be invisible to 
the naked eye, and can only be satisfactorily examined under 
pretty high powers of the microscope. For this reason, and 
because they are almost universally found in water, these crea- 
tures, often popularly called ‘“‘ animalcules,” are almost unknown 
to the majority of people. Some few, however, attain a large 
size, and of these the sponges are familiar examples. The mi- 
croscopical forms of the Protozoa swarm in most stagnant pools, 
and in all waters charged with organic matter so as to afford 
them food. Every worker with the microscope is familiarly 
acquainted with them, and they exhibit phenomena which in 
many cases render them objects of the highest interest. From 
their low position in the animal scale, it arises that the Protozoa 
are mainly characterised by the absence of organs and struc- 
tures which occur in higher beings, and they possess few 
positive characters by which they can be distinguished. 
The Protozoa may be defined as animals, generally of very 
minute size, composed of a nearly structureless jelly-like substance 
(called “sarcode”), showing no composition out of distinct 
