THE BATH SPONGE AND SOME ALLIES 285 



of the osculum. Many of the sponges are really colonies 

 produced by budding around the base of a single individual. 



Some of the spaces within a sponge are lined with spe- 

 cialized cells which were part of the endoderm during em- 

 bryonic development. These endoderm cells are furnished 

 with long'thread-like appendages called flagella. When these 

 flagella vibrate they force the water out through the oscu- 

 lum and thus draw in a fresh supply of water through 

 the pores in the surface of the body. Food particles con- 

 tained in the water are taken directly into the protoplasm 

 of the endoderm cells. Digestion takes place inside the 

 cells, not in a digestive cavity such as has been described 

 for all other free-living animals treated before in this text. 



Reproduction. Reference has already been made to the 

 fact that budding occurs in the sponges. They also repro- 

 duce sexually. Eggs and sperms are produced in the mature 

 sponge. Fertilization takes place inside the body of the 

 parent. The egg develops into an ovoid larva covered with 

 flagella. After leaving the body of the parent the larva 

 swims about for a while, finally settling down on some 

 solid object, where it transforms and grows into an adult 

 sponge. 



Fresh-Water Sponges. The sponge of Fig. 148 (Hetero- 

 meye'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 



