FRESH-WATER SPONGE AND SOME ALLIES 279 



phylum Porif'era, including only a single class. It is evident 

 that the Porifera are lower in organization than the Ccelen- 

 tera, for the members of the phylum do not show indications 

 of muscle-cells, of nerve-cells, or of sense-organs. Neither are 

 there special organs of offense. Protection is afforded by the 

 spicules and the characteristic odor. Sponges are the lowest 

 animals which reproduce by eggs and spermatozoa. In devel- 

 opment two germ-layers are present, ectoderm and endoderm, 

 and a middle undifferentiated layer called the mesoglwa. 



