Porifera. 311 



form a coating on logs and rocks, usually greenish or 

 yellowish green. In still water they branch and may 

 reach considerable height, but in swift streams form a low, 

 spreading mat. They are of no commercial value. Some- 

 times in reservoirs supplying drinking water they give an 

 unpleasant taste to the water. 



Relations of Sponges to Other Animals. None of the 

 larger animals eat sponges. This may be due to the pres- 

 ence of the sharp spicules, or to an unpleasant taste or 

 odor. Many small animals bore into or crawl into them, 

 some for a safe hiding place, doing no direct injury to 

 their host. Others are perhaps parasitic. While no 

 sponge is a parasite, some injure shells, as oysters, by bor- 

 ing into them. Certain sponges are found only on the 

 shells inhabited by hermit crabs, where they pay for their 

 transportation by concealing their bearer. 



Reproduction and Development of Sponges. Sponges 

 have two ways of multiplying, by budding and by eggs. 



In budding, a group of cells, called a " gemmule," is 

 formed, which becomes detached, and develops into a 

 sponge. Fresh-water sponges form these gemmules in the 

 fall. The gemmules lie dormant over winter, and begin 

 growth in the spring. 



Sponges produce eggs. These eggs are cells, which are 

 produced by the middle layer of the sponge (mesoderm). 

 Other cells, called sperm cells (or sperms), are produced 

 by some other part of the mesoderm, or perhaps by the 

 mesoderm of another sponge. After the egg cell is ferti- 

 lized by the sperm cell, it begins to divide, and forms a 

 number of cells which cohere. Part of these cells have 

 cilia, and by their vibration the embryo sponge swims 

 about, until finally it settles, attaches itself, and remains 

 fixed the rest of its life. While it is small and free, the 



