ctoderm (_h_) is encrusted with
grains of sand, above the sponge spicules.]

Here, then, we leave the lower forms of Metazoa in their condition of
permanent gastrulae. They differ from the transitory stage of other
Metazoa only in being enormously larger (owing to greatly further
_growth_, without any further _development_ as to matters of fundamental
importance), and in having sundry tentacles and other organs added later
on to meet their special requirements. The point to remember is, that in
all cases a gastrula is an open sac composed of two layers of cells--the
outer layer being called the ectoderm, and the inner the endoderm. They
have also been called the animal layer and the vegetative layer, because
it is the outer layer (ectoderm) that gives rise to all the organs of
sensation and movement--viz. the skin, the nervous system, and the
muscular system; while it is the inner layer (endoderm) that gives rise
to all the organs of nutrition and reproduction. It is desirable only
further to explain that gastrulation does not take place in all the
Metazoa after exactly the same plan. In different lines of descent
various and often considerable modifications of the original and most
simple plan have been introduced; but I will not burden the present
exposition by describing these modifications[15]. It is enough for us
that they always end in the formation of the two primary layers of
ectoderm and endoderm.

[15] The most extreme of them is that which is mentioned in the last
foot-note.

The next stage of differentiation is common to all the Metazoa, except
those lowest forms which, as we have just seen, remain permanently as
large gastrulae, with sundry specialized additions in the way of
tentacles, &c. This stage of differentiation consists in the formation
of either a pouch or an additional layer between the ectoderm and the
endoderm, which is called the mesoderm. It is probably in most cases
derived from the endoderm, but the exact mode of its derivation is still
somewhat obscure. Sometimes it has the appearance of itself constituting
two layers; but it is needless to go into these details; for in any case
the ultimate result is the same--viz. that of converting the Metazooen
into the form of a tube, the walls of which are composed of concentric
layers of cells. The outermost layer afterwards gives rise to the
epidermis with its various appendages, and also to the central nervous
system with its organs