36 ANIMAL LIFE 
shore where sponges are found can not observe the struc. 
ture and life history of living specimens. There are, how- 
ever, among the thousand and more kinds of sponges a few 
kinds that live in fresh water, and these are so widely 
spread over the earth that examples of them can be found 
in almost any region. They belong to the genus Spongilla, 
and thirty or more species or kinds of Spongilla are known. 
In standing or slowly flowing water, Spongilla grows erect 
and branching, like a shrub or miniature tree; in swift 
water it grows low and spreading, forming a sort of mat 
over the surface to which it is attached. Reproduction 
takes place very actively by the process of budding. The 
budded-off gemmules are spherical in shape, and the cells 
of each gemmule are inclosed in an envelope composed of 
siliceous spicules of peculiar shape. These gemmules are 
formed in.the body substance of the parent sponge toward 
the end of the year, and are set free by the decaying of 
that part of the body of the parent sponge in which they 
lie. They sink to the bottom of the pond or brook, and 
lie there dormant until the following spring. Then they 
develop rapidly by repeated division of the cells and 
growth. ; 
It is not the purpose here to describe the many and 
interesting kinds of sponges which inhabit the ocean. The 
sponge of the bathroom is simply the skeleton of a large 
sponge or group of sponges. The skeleton here is not 
composed of lime or silica, but of a tough, horny substance, 
which is secreted by cells of the mesodermal layer of the 
body wall of the sponge. This substance is called spongin, 
and is a substance allied to silk in its chemical composi- 
tion. All the commercial sponges, the spongin skeletons, 
belong to one genus—Spongia. These sponges grow espe- 
cially abundantly in the Mediterranean and Red Seas, and 
in the Atlantic Ocean off the Florida reefs, and on the 
shores of the Bahama Islands. The sponges are pulled 
up by divers, or by means of hooks or dredges. The 
