FRESH-WATER SPONGE. 157 



FRESH- WATER SPONGE. 



Fresh-water sponges may be found in clear water 

 in most parts of the United States. They grow 

 attached to submerged logs, stones, or stems of bushes. 

 One of the most common forms is Myenia fiuviatilis. 



1. Observe the sponge mass as a whole. Notice 

 its lack of definite shape. 



2. See the large openings in the surface of the 

 sponge. These are the oscula, or excurreut pores. 

 The currents of water come out of these pores. 



3. See the clusters of spicules arouud the oscula. 

 Observe also the pointed spicules projecting above 

 the surface. 



4. Cut through the sponge and look at a cross sec- 

 tion. Is there more than one layer of sponge sub- 

 stance? Does the sponge grow year after year? 



5. Look in the substance of the sponge for little seed- 

 like bodies. These are gemmules,or reproductive bodies. 



6. Where are the gemmules? Do they form a 

 layer at the base? Are they scattered through the 

 substance of the sponge? 



7. Examine the sponge as well as you can for the 

 spicules. 



8. Observe the color of the sponge. Is it green? 

 Did it grow in the sunlight? 



9. Spicules of a sponge may be separated from the 

 sponge substance by boiling first in sulphuric acid, 

 then adding, cautiously, nitric acid. The acid may 

 be removed by adding much water and allowing it to 

 settle, and repeating this process until all the acid 

 has been washed away. 



Commercial sponges consist of the skeletons of 

 certain kinds of marine sponges. Tlie fibers of these 

 skeletons are composed of a substance called spon- 

 giolin, which is very much like cliitin. Spongiolin is 

 found also in fresh -water sponges, but it is small in 



