212 



PRACTICAL ZOOLOGY 



and carbon dioxide and other excretory substances are given 

 off by it. 



FIG. 127. Types of sponges. 



A, fresh-water sponge ; B, a simple colonial marine sponge ; C, a simple solitary 

 marine sponge ; D, a bath sponge. (After various authors.) 



Reproduction. New individuals are produced by budding, 

 by the formation of gemmules, or by means of fertilized eggs. 

 Gemmules (Fig. 129, C) are little balls of cells that are formed 



