STRUCTURE AND FUNCTION 51 



2. A STUDY OF SPONGES 



To show how cells loosely associated may work together 

 Materials. 



The simplest of the many-celled animals are the sponges, 

 which, with one exception, are salt-water forms. That 

 one, the spongilla, is not easily found and is very difficult 

 to maintain in the laboratory. For these reasons the ma- 

 terial for this study is very meager, except at the seashore, 

 and much of the work must be done from diagrams and 

 reference books. Small simple preserved sponges and 

 complex toilet sponge skeletons will also be used. 



Definitions. 



Body wall, the outer wall in bodies of the many-celled 

 animals. 



Central cavity, the cavity surrounded by the body wall 

 in the simpler many-celled animals, as in the sponges. 



Canals, channels through the body walls of sponges. 



Inhalent pores, the outer ends of the canals. 



Ostia, the inner ends of the canals. 



Osculum, the large opening of the central cavity, at the 

 distal end of the sponge. 



Spicules, tiny needles of mineral substance found in the 

 walls of many sponges. 



Fibers, flexible threads of homy material found in the 

 walls of many sponges. 



Endoderm cells, cells lining the canals. They have cilia 

 or flagella (projections larger than cilia). 



Ectoderm cells, cells covering the outside of sponges and 

 some other animals. In sponges it is believed that endo- 

 derm and ectoderm cells are able to exchange positions and 

 functions. 



