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MAN IN THE MODERN  WORLD
increasing relative surface—they are flattened, or cylindrical, or, like
Foraminifera, have much of their substance in the form of a network
of fine living threads, or possess long thin processes, like nerve-cells.
With many-celled animals, similar considerations still hold good.
Food must be absorbed from a surface—the surface of the intestine.
In small forms, enough surface is provided by a straight, smooth tube,
but this would never work in larger animals. To get over the diffi-
culty, all sorts of dodges have been adopted. In large flatworms, the
whole gut is branched; in large Crustacea like lobsters and crabs,
absorption mostly goes on in the feathery e liver,' which provides
thousands of tubes instead of one; in the earthworm, the absorptive
surface of the intestine is nearly doubled by a projecting fold; in our-
selves, not only is the effective inner surface of the intestine multiplied
many times by the myriads of miniature finger-like villi, but the in-
testine itself is coiled; and in some herbivores the coiling is pro-
digious. Among lower animals without a fixed adult size, the period
for which rapid growth can continue must often depend upon the
inherited construction of the intestine. For instance, in flatworms, if
the gut is a simple tube, increase of bulk rapidly brings down the
relative surface, and the animal while still quite small can only eat
enough to keep itself going, but not to grow; while if the gut is
elaborately branched, growth will not be slowed down until a much
larger bulk has been reached.
The same sort of arguments apply equally well to other processes,
such as respiration and excretion, whose amount depends on amount
of available surface. In small animals gills can be unbranched; in
big ones they must be feathery. Large vertebrates like us could not
breathe if their lungs were not partitioned off into millions of tiny sacs.
The coiling and multiplication of kidney tubules in large animals are
equally necessary. An embryo frog excretes by means of three pairs
of kidney tubules. An adult frog would die from accumulation of
waste substances if he possessed only six large tubes of equivalent
proportions, even if their walls remained thin enough for secretion;
what he needs is many thousands of small tubules.
When the animal is small, no transport system is necessary to get
the food or water or oxygen to the cells from the original absorptive
surface; all goes well by diffusion alone. But bulk brings difficulties
here too. The flatness of the larger flatworms is partly due to the need
for having every cell near enough to the surface to be able to get
oxygen by diffusion. The elaborate branching of their intestines and
all other internal organs is needed to ensure that no cell shall be more
than a microscopic distance away from a source of digested food,
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