Excretion - 371 



spaces of the mesoderm — and drive the re- 

 sulting solution of wastes out of the body, 

 via the excretory ducts. 



In Annelida, such as the earthworm, the 

 mesodermal tissues are much more abundant 

 and highly differentiated; also there is a well- 

 developed blood system. The excretory or- 

 gans of the earthworm and other Annelida 

 are the nephridia, of which there are usually 

 two in each segment of the body (Fig. 20-3). 

 Each nephridium is a long, highly coiled 

 tube, which leads from the body cavity of 

 one segment to the external surface of the 

 succeeding segment of the body. At the co- 

 elolic end of the nephridium there is a cilated 

 funnel-shaped opening, the nephrostome. 

 The lumen of the nephridium — throughout 

 most of its length — is also lined with cilia. 

 Moreover, there is a network of capillaries 

 (not shown in the figure) that comes into 

 intimate contact with the glandular walls of 

 the nephridium. 



CIRCULAR MUSCLES 



ENDODERMAL 

 EPITHELIUM 



NEPHRIDIUM 



CIRCULAR 

 MUSCLE 



ECTODERMAL 

 EPITHELIUM 



CUTICLE 



NEPHROSTOME 



NERVE 



EXCRETORY PORE 



The functioning of the nephridial tubules 

 has not been studied very adequately. How- 

 ever, the nephridia tend to resemble the kid- 

 ney tubules of higher animals (p. 372); and 

 it may be supposed that they function in a 

 somewhat similar fashion. The cilia main- 

 tain a flow of the body fluid — from the 

 coelomic cavity, through the tubule, toward 

 the external opening. The coelomic fluid 

 contains glucose and other useful substances, 

 as well as metabolic wastes, and probably the 

 glandular cells in the wall of the nephridium 

 — like the cells in the wall of a kidney tubule 

 — extract glucose, water, and other useful 

 substances from the fluid while it passes to- 

 ward the outside. In this way useful com- 

 ponents of the body fluid are returned to the 

 blood in the capillaries of the tubule; and 

 waste products, due to a reabsorption of 

 water, are more concentrated by the time the 

 excretory fluid is voided at the surface of the 

 body. 



DORSAL BLOOD VESSEL 



LONGITUDINAL 

 MUSCLE 



PERITONEUM 



LONGITUDINAL 

 MUSCLES 



BRISTLES 



PERITONEUM 



VENTRAL BLOOD 

 VESSEL 



LATERAL- NEURAL 

 BLOOD VESSEL 



SUBNEURAL BLOOD 

 VESSEL 



NERVE CORD 



Fig. 20-3. Diagrammatic cross section of an earthworm. Note particularly the nephridium, an excretory 

 tubule that originates at the nephrostome and terminates at the excretory pore. 



