462 EVOLUTION OF ANIMALS Part V 



the flagellate chambers. A considerable bulk of sponge should be put through 

 the cloth. There must be a sufficient number of cells, especially food-carrying 

 amebocytes, or regeneration will not occur. Collared cells will not collect 



Fig. 22.8. Germinating gemmules of Spongilla. 

 The young colonies have surrounded the capsules of 

 the gemmules from which they grew. Readily reared 

 on glass. 



except about amebocytes. If bodies of two different species of sponges are put 

 through the bolting cloth, their cells may at first intermingle, but soon those of 

 each kind congregate by themselves. 



Fresh-water Sponges 



All fresh-water sponges are classified in the family Spongillidae, of which 

 there are about 20 American species (Fig. 22.6). They grow in clean water, in 

 ponds, lakes, and streams, upon stones, the undersides of lily pads, and sub- 

 merged stems and sticks. When they are in full light they are often colored 

 green by Zoochlorellae, the unicellular algae, within their cells. Many are 

 annual growths, germinating from gemmules in the spring, reaching full size 

 in mid-summer and dying away toward autumn except for the new crop of 

 gemmules (Fig. 22.7). Fresh-water sponges are inhabited by a few minute 

 residents, not large or as numerous as those that live in marine sponges. Among 

 them are the larvae of Spongilla flies that puncture the sponge cells and suck 

 up the protoplasm. They are about a quarter of an inch long and match the 

 sponge color exactly. The best way to find them is to watch for what appear to 

 be bits of sponge moving about through the sponge colony. Compared with 

 marine sponges fresh-water ones are small, scanty growths. Nevertheless, in 

 reservoirs they may spread through the water pipes unless the water is chemi- 

 cally treated. This has occurred in the water systems of more than one large 

 city. 



