We A. KIDD, ESQ., M.D., ETC., ON PROTECTION AMONG ANIMALS. 147 
torrid, temperate, and frigid zones, in rivers, lakes, and 
ponds, on the earth and in its crust, and in the bodies of 
ether animals. Here, then, in so motley an assemblage of 
environments, is a great field in which may be displayed 
those multiform methods of protection which I, for one, 
cannot dissociate from the action of mind and will. 
In considering this subject of protection one must some- 
what enlarge the ordinary meaning of the word, in accordance 
with the common maxim in the affairs of nations that the 
surest way to peace and protection is to be prepared for war, 
“St vis pacem bellum para.” This is denied by certain 
prominent. thinkers of philanthropic bent, but it obtains much 
support among the higher members of the animal kingdom. 
We shall see many of these, in which the methods ot 
protection are almost altogether active rather than passive 
in character, and in which formidable weapons of offence 
supply adequate protection to their possessors, otherwise 
feebly provided with passive methods of protection. 
The varying modes in which the need for protection 
among animals has been met afford contrivances so diverse 
from one another as the outer thickened and hardened 
layer of an amoeba, protecting the softer internal parts, 
the skull of man protecting his delicate brain, or the oil with 
which an aquatic bird preens its feathers for protection 
against moisture and cold. 
The two most important divisions of the animal kingdom 
are the invertebrates or non-chordate animals and vertebrates 
or chordate animals. Of the former the Protozoa form 
the lowest group. This great group of small creatures 
comprises those which have no specialization of functions 
for the different cells of their bodies. They may be called 
one-celled by reason of the fact that the protoplasm of 
which they consist discharges indifferently the functions of 
nutrition, reproduction, and relation. The rest of the animal 
kingdom is marked off from the Protozoa as Metazoa, 
and includes creatures which range from a sea anemone toa 
man. 
Among the Protozoa the need for protection is slight, as 
they are aquatic and mostly microscopic in size. But in the 
amoebee, almost the lowest Protozoa, there is a provision 
by which the outer layer of protoplasm, or ectosarc, becomes 
relatively hardened, and serves to protect the inner portions. 
In somewhat higher Protozoa a substance called “ chitin” is 
produced on the surface. It is a hard, horny, organic tissue, 
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