Bespiration and Cell Energy. 
413 
Eespiration and Evolution. 
Eespiration will now be reviewed throughout animal and plant life, 
taking respiration to mean simply the interchange of gases in each living 
cell. Facility for the supply of oxygen to the tissues must therefore be a 
concomitant of complexity in structure. Thus every step in evolution 
must in reality be preceded by or closely connected with some provision 
for supplying oxygen to every cell in the organism. 
In Amoeba or any unicellular animal or plant respiration is usually 
very simple. Such organisms can only live in water. Now all water 
contains air and therefore oxygen dissolved in it to a slight extent, and 
this is the only oxygen which is available for aquatic life of any kind, 
whether plant or animal. The oxygen of composition of water is never 
utilised in respiration. For such water organisms, therefore, the surround- 
ing medium contains a practically unlimited supply of oxygen, and is so 
vast that the carbon-dioxide put out into the medium is quickly removed 
from the neighbourhood of the organism. Whether the interchange of 
gases takes place only at the surface of such organisms is not known, 
but it is hardly probable, considering that the vital functions of protoplasm 
are carried on throughout its whole mass. 
In the simpler multicellular animals and plants where there is some 
differentiation of the tissues the massing of the cells is never so great as 
to cut off the internal cells from contact with water. In Volvox, which 
is a colony of unit cells, the cells are arranged in the shape of a hollow 
sphere, the space inside being filled with water. In Hydroids, Sponges, &c., 
there is also an internal cavity in which a flow of water is continually kept 
up, and this constitutes not only a respiratory current for internal cells 
but also serves as the nutritive current. In star-fishes, where more dif- 
ferentiation has occurred and the internal cells are further removed from 
the exterior and where there is a special nutritive canal, respiration has 
been arranged for by a specially contrived system of water-vessels through 
which water is kept continually moving. This system, which is also used 
in the peculiar locomotion of the animal, is known as the water vascular 
system. 
So far, then, the living cells of the organisms considered receive their 
supply of oxygen by contact with the medium in which they live. Animals 
and plants of more complex structure have now to be considered. Pro- 
vision for respiration must always keep pace with increase of complexity 
of structure. 
In plants, contact of the living cells with the surrounding medium 
either of air or water is relied on. Of course, respiration is practically 
absent from seeds, dried-up moss plants, &c. so long as they remain 
desiccated. In mosses the leaves are seldom more than one cell in thick- 
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