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. 

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



