SPONGES 



an open osteole at the top. If a bit of finely powdered car- 

 mine or starch is scattered on the colony, the particles will be 

 carried down into the osteoles and will soon be seen coming 

 out of the larger oscula. If the colony, glass base and all, is 

 taken from the water for a moment and held up to the light, 

 the translucent spots and streaks will tell where chambers 

 and canals have already formed within it (PI. VI, 2). When 

 a colony is five or six days old its spicules will prick through 

 the filmy covering of its transparent border (Fig. 86). 



Identification. — Fresh water sponges have been separated 

 into two groups according to the structure of their gem- 

 mules ; one contains those in which the opening of the gemmule 

 is a simple hole, the other those which have it at the end of a 

 short tube; they have been further classified by their spicules. 



Although traits and general form are helpful, for the final 

 identification of fresh water sponges microscopic study is 

 necessary. Ward and Whipple's "Fresh Water Biology" 

 and Pratt's "Manual of Invertebrate Animals" both contain 

 illustrated keys to the fresh water sponges. 



Ill 



