FIELD BOOK OF PONDS AND STREAMS 



plant cells, the zoochlorellas, which grow within their own 

 cells. It is a common occurrence to find green sponge colonies 

 on the upper surfaces of submerged logs (PI. X) and yellowish 

 or nearly white ones on the under side. 



Distribution. — Only 40 or 50 species of fresh water sponges 

 are known in North America. Two of the commonest, 

 Spongilla fragilis and Spongilla lacustris, are found in the 

 eastern states and as far west as Kansas. There are fewer 

 records of sponges in the southern states and those beyond 

 the Mississippi River but this is probably due to lack of study 

 rather than a scarcity of sponges. 



Fig. 84. — Fresh water sponge in winter, gemmules 

 held among interlacing spicules. 



Season. — Colonies are at the height of their growth in July 

 and August. In the early fall most of them begin to shrivel, 

 by October *id November the}^ are dead, and only the gem- 

 mules or winter buds remain, held among the interlocked 

 spicules. These are»the only parts of sponges which live over 

 the winter; in March and April young colonies begin to grow 

 from them, first appearing as tiny flecks of white upon sub- 

 merged stones and twigs. 



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