26 ANIMAL FORMS 
is absorbed by all parts of the body in contact with the 
water. 
27. Skeleton of sponges.—When it is remembered that 
the protoplasm composing the cells of the sponge has about 
the same consistence 
as the white of egg, 
it will be readily un- 
derstood why the 
greater number of 
sponges possess a skel- 
eton. Without such 
Fig. 14.—Portion of wall of sponge, showing three a supp ort the larger 
layers. e, outer layer ; é, inner layer, consisting 
of collared cells’; m, middle layer, consisting of globular or branched 
irregular cells, among which are the radiate spic- forms could not ex- 
ules and egg-cells. 5 s 
ist, and even in the 
smaller members there would be danger of a collapse of the 
body walls and consequent stoppage of the food supply, 
owing to the closure of the pores. So in all but a very few 
thin or flat forms a skeleton appears in the young sponge 
almost before growth 
has fairly begun, and 
this increases with the 
body in size and com- 
plexity. It is formed 
by the activity of the 
cells of the middle layer, 
and may be composed 
either of a lime com- :, 
ae ae a Fia. 15.—Different types of sponge spicules, 
horn-like substance resembling silk, or these may exist in 
combination in certain species. When consisting of either 
of the first-named substances it is never formed in one 
continuous piece, but of a vast multitude of variously shaped 
crystal-like bodies termed spicules (Fig. 15). These occur 
everywhere throughout the body, firmly bound together 
