60 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



the inhalant and radial canals, is the soft gelatinous-like 

 layer, the mesoderm, that is composed of various kinds of cells. 

 Some of these are concerned in the formation of the spicules 

 that make up the framework, some are concerned with diges- 

 tion and some with reproduction. Two or three kinds of spic- 

 ules may be found in the mesoderm of Grantia. They are 

 composed of carbonate of lime, and are very brittle when dry. 

 The framework of the commerical sponges is composed of 

 exceedingly fine flexible fibers of a horny substance called 

 spongin. This is the part of the animal that we commonly 

 know as the sponge in the market. 



Grantia reproduces itself in two different ways. Small buds 

 sometimes appear on the external surface of the body which 

 develop into small individual sponges. These gradually 

 increase in size and finally break away from the parent and 

 attach themselves to some other substance. In the more 

 complex sponges these buds do not break away, but remain 

 attached so that in time there is built up a complex sponge 

 colony. 



Besides this method of reproducing asexually, that is, with- 

 out the union of two kinds of germ cells, all sponges have a 

 mode of sexual reproduction. The male, or sperm, cells and 

 the female, or egg, cells are produced in the same individual. 

 The sperm cells when ripe are cast into the water and swim 

 about until they come in contact with egg cells which they 

 fertilize. From these fertilized egg cells sponge embryos 

 develop which, after they have reached a certain stage, swim 

 away by means of many cilia which cover their bodies and 

 finally attach themselves to some substance where they remain 

 the rest of their lives. 



Sponges of Commerce. All sponges have the same general 

 type of structure but most of them branch extensively and 

 form immense colonies with innumerable canals and cavities 

 making altogether a complicated network. The sponges that 

 we use are really only the skeletons of some of these great 

 colonies with all of the soft parts dried or squeezed or washed 

 out. Bleaching powders or acids are sometimes used to 

 lighten the color, but they are apt to injure the fibers. The 



