192 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



of the skin, where the oxygen is picked up and combined with the 

 hemoglobin, to be later released in the cells of the body where work 

 is done. Carbon dioxide and wastes are here taken up by the blood 

 and carried back to the skin and to the nephridia or excretory organs. 

 One can easily demonstrate the network of tiny capillaries in the skin 

 where this exchange takes place. 



Organs of Excretion 



The paired nephridia are essentially coiled tubular organs, made 

 up of a ciliated funnel or nephrostome that opens into the coelom, 



a thin ciliated glandular 



^-^epttcin 



like region 



tube, that loops on itself 

 about three times, and a 

 pore, the ncphridiopore, 

 through which the excre- 

 tory products pass to the 

 exterior. Some excretory 

 materials are probably 

 taken directly from the 

 coelomic fluid by means 

 of the currents caused 

 by the cilia, while other 

 wastes may be taken 

 directly from the blood- 

 capillaries which cover 

 the surface of the glan- 

 dular tubules. One characteristic feature of the nephridium is that it 

 always passes through the septum separating two segments. 



A nephridium of an earthworm. Trace the 

 passage of fluid from the coelom to the exterior of 

 the worm. Note the ciliated surface of the neph- 

 rostome. What is its function .!^ (After Wolcott.) 



The Muscles and Their Work 



Movement is brought about by muscular contraction. As an 

 earthworm crawls, a wave of contraction from the posterior toward 

 the anterior appears to move up the body of the worm. A careful 

 examination shows that movement is brought about by the contrac- 

 tion and relaxation of two opposing groups of muscle fibers and by 

 the movement of the rows of setae on the ventral surface. The 

 muscles are arranged in two layers just under the skin, an outer 

 circular layer running around the body and an inner longitudinal 

 layer. When the worm lengthens, the longitudinal muscles relax 



