66 ZOOLOGY. 



the nitrogenous wastes, urea and uric acid. In organisms in 

 which there is no regular blood system, these waste products 

 may be carried directly from the tissues to the surface by a 

 system of tubes beginning as capillaries. In the majority of 

 animals the canals (nephridia) pass from the body cavity to 

 the exterior. These are seen in a simple condition (Fig. 33) 



FIG. 33. 



FIG. 33. Diagram of a nephridium (simple kidney tubule) of a Segmented Worm. 

 b., b'., blood vessels; c, ccelome; d, duct of the nephridium; e, external opening; cf, 

 ciliated funnel opening into ccelome; gl, glandular or secreting portion; s, septum; 

 W, body wall composed of longitudinal muscle fibres, circular fibres, and epithelial 

 layer; w, wall of gut. 



Questions on the figure. Judging from its relation to the coelome, to 

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

 function for the nephridium? 



in the segmented worms. For a more complicated condition 

 see unsegmented worms (Figs. 86-88). The kidneys of 

 higher forms are considered to be derived from these. In the 

 higher animals the kidneys have a special blood supply and 

 the waste products are extracted from the blood while in the 

 kidney. 



95. 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 cells together 



