THE FRESH- WATER SPONGE AND SOME ALLIES : PORIFERA 



The unending shapes of plants, the rainbow's varied hues, 

 All these, the lowly sponge on ocean's bed renews. 



The Fresh-Water Sponge. The sponge illustrated in Fig. 

 136 (Heteromeye'nia ry'deri) is found quite generally in ponds 

 and quiet brooks, at least as far west as the Mississippi River. 

 It grows on the under side of overhanging submerged rocks, 

 and on dead sticks and leaves. The largest specimens are 

 not usually more than one inch across. Each mass of sponge 

 clings flat against the supporting substance, and seldom is 

 more than one eighth of an inch thick. The sponge yields 

 slightly on being pressed with the finger. The surface looks 

 rough, but to the touch it is smooth. The color is grayish; 

 occasionally specimens are found with a part or all of the 

 mass green. This is due to the presence of a green alga 

 growing in the sponge. 



The first thing that attracts one's attention in the drawings 

 of the fresh-water sponge is the many small, circular holes, 

 oscula (sing, osculum, a little mouth), scattered over the surface. 

 They are openings through which the waste, the unused food, 

 and water are expelled from the sponge. Everywhere between 

 the oscula are numerous very small holes, dermal pores (com- 

 pare Fig. 137, 5), which open into quite large subdermal cham- 

 bers. Leading from the subdermal chambers are short canals 

 called incur rent canals. These lead to cavities lying deeper 

 in the sponge, the ampullce (Fig. 137, c), which are lined 

 with cells that bear flagella (lashes). These cells are called 

 collar-cells, on account of their having a collar-like membrane 



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