to a human society. The freshwater polyp
_Hydra_, like the foregoing illustrations, is one whose structure has
already been discussed in the earlier chapters, but now we may use it for
an analysis of another series of biological phenomena. Its sac-like body
consists of two cell-layers; the outer one is concerned primarily with
offense and defense, while the inner layer is made up of digesting or
nutritive elements. The essential cells concerned solely with reproduction
lie below the outer sheet. Comparing this animal with an association like
_Volvox_, we discover the same differentiation into immortal germ-elements
and mortal cells, concerned respectively with the _Hydra's_ racial
existence and with its individual life; but far-reaching changes have come
about in the biological relationships of the second class of cells. In
describing the new phenomena it is absolutely necessary to employ the
terms of human social organization, because the _Hydra's_ body is a true
colony of diverse cells in exactly the same sense that a nation is a body
of human beings with more or less dissimilar social functions.

To begin with the differentiation into ectoderm and endoderm, the organism
is comparable to a human community made up of military and agricultural
classes. The cells of the former group protect themselves and the feeding
elements also, while the units of the second defenseless type devote
themselves to the task of provisioning the whole community, giving
supplies of food to the defenders in exchange for the protection they
afford; each kind needs the other, and each performs some distinctive task
for the other as well as for itself. But the parallel thus drawn need not
stop here. In the case of the outer layer, the cells are mostly flat
covering elements that are the first to be torn off and injured when the
animal is attacked. Scattered about among them are sense-cells standing
like sentinels with delicate upright processes which receive stimuli from
without the sense-cells transmit impulses to the network of nerve-cells
below, which is a counterpart of the signal corps of an army, keeping all
parts of the whole organization in communication with one another. Most
wonderful of all are the stinging-cells of the outer layer; these produce
a flask-shaped, poisoned bomb which is discharged by the convulsive
contraction of the cell itself so as to stun and injure the enemy or prey.
The bomb-throwing cells die immediately after th