GENERAL ANIMAL FUNCTIONS. 85 



elimination of indigestible substances from the digestive 

 tract is not excretion, in the sense in which we are using 

 the term. Why? 



91. The Skeletal System and its Functions. The cells 

 of the body frequently excrete from themselves materials 

 which are of no further use to the protoplasm, but which 

 may come to serve some important function if deposited 

 around or among the cells. The cell walls themselves 



FIG. 13. Diagram of an excretory organ (nephridum) of a segmented worm, b, 

 blood vessels; c, coslom; d, duct of nephridium ; e, external opening; /, funnel, opening 

 into coslom; gl, glandular or secreting portion; s, septum; w, body wall; w, wall of gut. 



Questions on the Figure. Judging from the relation of the neph- 

 ridium to the coslom, to the blood vessels, and to the outside world, 

 what would seem a reasonable function for this organ? What do 

 the septa accomplish? 



may be of such substances. The intercellular substance 

 among the cells (as the hard part of bone), and sheets of 

 protective material on the surface of the body (as in the 

 cuticula of the locust or the shell of the clam) are ex- 

 amples of this. 



Such secretions serve a very important passive function 

 in protecting the soft parts and in furnishing hard parts, 

 or skeletons, for the attachment of muscles. In such 

 external skeletons as that of the crayfish the same 



