THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 83 
may become so interwoven with each other as to form a 
very complex group. A sponge is composed of many cells 
arranged in three layers—that is, the body of a sponge is a 
cylinder closed at one end whose wall is composed of three 
layers of cells. The outer layer of 
cells is called the ectoderm, and the 
cells composing it are flat and are 
all closely attached to each other. 
The inner layer is called the endo- 
derm, and its cells are thicker than 
those of the ectoderm ; they are 
also closely attached to each other. 
Sometimes they are provided with 
flagella like the flagellate Protozoa. 
The flagella are, however, not for the 
purpose of locomotion, but for creat- 
ing currents in the water, which 
bathes the interior of the open cylin- 
drical body. The middle layer, 
called the mesoderm, is composed of 
numerous separate cells lying in a 
jelly-like matrix. From these meso- 
derm cells fine needles or spicules 
of lime or silica often project out 
through the ectoderm. These mi- 
nute sponge spicules are of a great 
variety of shapes, and they form a 
sort of skeleton for the support of 
the soft body mass. All over the 
outer surface of the body are scat- 
tered fine openings or pores, which 
lead through the walls of the body 
i cae 
Fig. 17.—One of the simplest 
sponges, Calcolynthus pri- 
migenius (after HAECKEL). 
A part of the outer wall is 
cut away to show the in- 
side. 
into the inner cavity. This cavity is of course also con- 
nected with the outside by the large opening at the free or 
apical end of the body. 
There is hardly any differentiation of parts among the 
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