634 



APPENDIX B 



usually a noncellular layer, the mesoglea. The mesoglea is a jellylike 

 substance, in which occur the germ cells and a skeleton of spicules or 

 spongin. Embryological data indicate that the two germ layers are not 

 homologous with the ectoderm and entoderm of higher phyla. Symmetry 

 radial or lacking. No metamerism. Possessed of a generalized body cavity, 

 the cloaca, reached from the outside by way of many small incurrent 

 pores and opening to the outside by a single large excurrent pore, the 

 osculum. Body cavity not homologous with that of the coelenterates, 

 which it superficially resembles. 



Fig. B.3. Representative sponges. Left, the spongin fiber skeleton of a Florida glove sponge, 

 Hippospongia. Right, the redbeard sponge, Microciona prolifera. (Courtesy U.S. Fish and 

 Wildlife Service and American Museum of Natural History, respectively.) 



Among the features peculiar to this group are the presence of pores in the body 

 wall, forming part of a canal system, and the presence in the gastral epithelium 

 of collared cells, the flagella of which create a current of water through the canals 

 that brings in food particles and oxygen and carries away waste products. 



Sponges are an aberrant group that belongs near the bottom of the animal 

 series and outside the main line of metazoan evolution; hence they are often 

 separated from the rest of the Metazoa and are called Parazoa. A very few species 

 of sponges live in fresh water. The majority, found in the shallow waters of the 

 ocean, are encrusting or weakly erect; those occurring in the deeper portions of 

 quiet seas include stemmed, foliate, and ramifying forms, such as finger and cup 

 sponges, the Venus'-flower-basket, and the bath sponge of commerce. Many 



