26 ANIMAL PHYSIOLOGY. 
if we exclude the doubtful case of the thread-cells (wrticating 
capsules). The animal breathes by the entire surface of the 
body ; nourishment passes from cell to cell, and waste is dis- 
charged into the water surrounding the creature from all cells, 
‘though probably not quite equally. All parts are not digestive, 
respiratory etc., to the same degree, and herein does it differ 
greatly from Ameeba or even Vorticella, though fullet knowl- 
edge will likely modify our views of the latter two and similar 
organisms in this regard, 
THE CELL RECONSIDERED. 
Having now studied certain one-celled plants and animals, 
and some very simple combinations of cells (molds, etc.), it will 
be profitable to endeavor to generalize the lessons these humble 
organisms convey; for, as will be constantly seen in the study 
of the higher forms of life of which this work proposes to treat 
principally, the same laws operate as in the lowliest living 
creatures. The most complex organism is made up of tissues, 
which are but cells and their products, as houses are made of 
bricks, mortar, wood, and a few other materials, however large 
or elaborate. 
The student of physiology who proceeds scientifically must 
endeavor, in investigating the functions of each organ, to learn 
the exact behavior of each cell as determined by its own inher-. 
ent tendencies, and modified by the action of neighboring cells. 
The reason why the function of one organ differs from that of 
another is that its cells have departed in a special direction 
from those properties common to all cells, or have become func- 
tionally differentiated. But such a statement has no meaning 
unless it be well understood that cells have certain properties in 
common. This is one of the lessons imparted by the preceding 
studies which we now review. Briefly stated in language now 
extensively used in works on biology, the common properties of 
cells (protoplasm), whether animal or vegetable, whether con- 
stituting in themselves entire animals or plants, or forming the 
elements of tissues, are these: The collective chemical processes 
associated with the vital activities of cells are termed its meta- 
bolism. Metabolism is constructive when more complex com- 
pounds are formed from simpler ones, as when the Protococcus- 
cell builds up its protoplasm out of the simple materials, found 
in rain-water, which make up its food. Metabolism is destruct- 
