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. Eeproduction 

 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 Eed 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 



