ECONOMIC IMPORTANCE 
157 
body of the parent sponge. A sperm enters the body of 
another sponge and when it finds an egg, fuses with it, thus 
forming the fertilized egg. The fertilized _ 
egg then begins to grow, and after a defi¬ 
nite period breaks away from the parent, 
moves about for a time, and then settles 
down, attaches itself, and grows into a 
mature sponge. The immature sponge 
has the power of locomotion, but the 
mature form has lost this power. Never¬ 
theless the sponge is an animal. 
Reproduction that comes about through 
the fusion of an egg and a sperm is called 
sexual reproduction. The other method 
of reproduction, called asexual reproduc¬ 
tion, also occurs among sponges. By 
this method, sponges form little buds or 
branches which develop into new sponges. 
135. Spongilla. —Spongilla (spunj-il'la) 
is a fresh-water sponge. At the approach 
of cold weather, certain reproductive 
bodies are formed, known as winter-cells, 
and these escape from the sponge. They settle down to the 
bottom of the pond or stream and remain dormant until the 
approach of warm weather, when 
they grow into new sponges. 
They have a thick protecting 
coat which enables them to resist 
unfavorable conditions. 
136. Economic Importance. — 
The spicules of the different 
sponges form a large part of their 
so-called skeletons. These spic¬ 
ules are, in some cases, com¬ 
posed of lime and form the limy 
Figure 146. 
The sponge begins 
in a single cell that 
divides into two, then 
into four. After a 
while the cells be¬ 
come arranged as in 
A and the young 
sponge swims about; 
as it grows it be¬ 
comes attached and 
changes shape as in B. 
Figure 147.— Sponge Spicules. 
Not all these different kinds are 
found in each sponge. 
