SORTING OF ANIMALS 3 
other microscopic plant or animal and gradually incloses it. 
Thus it is all mouth and all stomach. It keeps growing until it 
reaches a certain limit of size, when it subdivides and becomes two 
individuals, each one of which is exactly one-half the size of the 
original. The two are now the offspring, the parent having disap- 
peared into its progeny. 
3. Sponges.—As we ascend the scale of 
simplest animals we come to where there 
is a community of cells. Animals con- 
sist of either one ‘cell or of many 
cells. Sponges are an example of the 
latter. In their young state they swim 
about, but soon attach themselves to some 
solid substance on the bottom of the 
sea where they grow and develop. Each ani- 
mal is practically a stomach in form, yet each 
One or tHE Sim- cell gathers its own food and assimilates it, 
PLEST SPONGES : 
and each obtains the necessary fresh air 
from the water circulating about it. The sponges of com- 
merce are gathered by divers, who pull them up from the 
ocean beds by means of mechanical devices. Once re- 
moved from the water they soon die, only the soft skele-' 
ton remaining. This now becomes a valuable aid in 
man’s work. 
4. Hydra.—Higher in the scale of cellu- 
lar complexity comes the hydra, in which 
there is a division of labor. This simple 
animal has sensitive parts corresponding to 
nerve cells. A sticky substance is secreted 
which enables the animal to attach itself 
to stone or plant for temporary habitation. 
From tiny projections are thrown out tiny 
threads, which paralyze an animal that it ,..5, water 
uses for food. It has a mouth for introduc- POLYP 
a . . «_ a, expanded condi- 
ing food into the, body; and this food is tion; 's, contracted. 
