28 ANIMAL PHYSIOLOGY. 
with those of higher animals. The condition of things in such 
an animal as Amosba may be compared to a civilized commu- 
nity in a very crude social condition. "When each individual 
tries to perform every office for himself, he is at once carpenter, 
blacksmith, shoemaker, and much more, with the natural re- 
sult that he is not efficient in any one direction. A community 
may be judged in regard to its degree of advancement by the 
amount of division of labor existing within it. Thus is it with 
the animal body. We find in such a creature as the fresh-water 
Hydra, consisting of two layers of cells forming a simple sac, a 
slight amount of advancement on Amceba. It®external surface 
‘no longer serves for inclosure of food, but it has the simplest 
form of mouth and tentacles. Each of the cells of the internal 
layer seems to act as a somewhat improved or specialized Amoe- 
ba, while in those of the outer layer we mark a beginning of 
those functions which taken collectively give the higher ani- 
mals information of the surrounding world. 
Looking to the existing state of things in the universe, it is 
plain that an animal to attain to high ends must have powers 
of rapid locomotion, capacity to perceive what makes for its in- 
terest, and ability to utilize means to attain this when perceived. 
These considerations demand that an animal high in the scale 
.of being should be provided with limbs sufficiently rigid to sup- 
port its weight, moved by strong muscles, which must act in 
harmony. But this implies abundance of nutriment duly pre- 
pared and regularly conveyed to the bones and muscles. All 
this would be useless unless there was a controlling and ener- 
gizing system capable both of being impressed and originating 
impressions. Such is found in the nerves and nerve-centers. 
Again, in order that this mechanism be kept in good running 
order, the waste of its own metabolism, which chokes and poi- 
sons, must be got rid of—hence the need of excretory apparatus. 
In order that the nervous system may get sufficient informa- 
tion of the world around, the surface of the body must be pro- 
vided with special message-receiving offices in the form of 
modified nerve-endings. In short, it is seen that an animal as 
high in the scale as a mammal must have muscular, osseous 
(and connective), digestive, circulatory, excretory, and nervous 
tissues ; and to these may be added certain forms of protective 
tissues, as hair, nails, etc. 
Assuming that the student has at least some general knowl- 
edge of the structure of these various tissues, we propose to tell 
in a simple way the whole physiological story in brief. 
