THE GENERAL ANIMAL FUNCTIONS 73 



have a special blood supply and the waste products are extracted 

 from the blood while it is in the kidney. 



98. The Skeletal System and its Functions. — The cells of 

 the body frequently excrete from themselves materials which, 

 while no longer of use to the protoplasm, are not entirely re- 

 moved from the body by the blood and serve important passive 

 functions. These excretions or secretions may surround and 

 protect and give rigidity to the cell itself (e.g., cell-walls; 

 shells in the single-celled animals), may bind the softer cells to- 



FiG. 35- 



-W 



Fig. 35. Diagram of a nephridium (simple kidney tubule) of a Segmented Worm. 6., 6'., 

 blood vessels; c, coelom; d, duct of the nephridium; e, external opening; cf, ciliated funnel opening 

 into coelom; gl., glandular or secreting portion; s, septum; W, body wall composed of longitudinal 

 muscle fibres, circular fibres, and epithelial layer; w, wall of gut. 



Question on the figure. — ^Judging from its relation to the coelom, to the blood 

 vessels, and to the outside world what would seem a reasonable function for the 

 nephridium? 



gether into a resistant tissue (intercellular substance in bone, etc.), 

 or may be secreted at the surface of the organism as a whole 

 (cuticula in insects and shell in mollusks). The hard parts 

 serve primarily for the support and protection of the softer 

 tissues. Incidentally they come to serve a very important use 

 as points of attachment for muscles. The skeleton may be 

 external (the integumentary skeleton, or e.xoskeleton) as in 

 crayfish, or internal, as the endoskeleton of vertebrates. In 

 many instances both kinds of skeletal structures may occur 

 simultaneously, yet it is usually true that if the exoskeleton 

 is well developed the endoskeleton will be poorly represented. 

 Each has important advantages and limitations. To allow 

 motion as the result of muscular action the skeleton, if rigid. 



