120 GENERAL BIOLOGY 



though a component part of a single organism, has a 

 certain structural individuality of its own, and is 

 separated from its neighbors on either side by a 

 partition wall, the septum. The inclosed space, 

 the " body-cavity," is filled with a watery fluid and 

 is lined with a network of blood-vessels through 

 whose walls the circulating waste-products transfuse 

 into this fluid. In each segment there is found an 

 open-mouthed, trumpet-shaped tube, fringed with 

 strong cilia, which passes through the septum into 

 the cavity of the segment just behind, and after a 

 more or less convoluted course opens directly through 

 the body-wall of that segment to the exterior. 

 The action of the cilia creates a current in the fluid 

 which passes down the tube, and drains off to the 

 exterior the liquid contents of the body-cavity, laden 

 with wastes. Such an excretory apparatus is of 

 very widespread occurrence in various groups of 

 animals and is called a nephridium (plural, nephridia) . 

 Such a nephridium is found only in the most 

 simply organized annelids. In the higher annelids 

 we find the mechanism much improved by the elimi- 

 nation of unnecessary steps and the securing of greater 

 economy of energy and increased efficiency. Thus 

 in the earthworm, branches of the blood-vessels 

 surround portions of the nephridial tube, and the 

 major portion of the excreted substances transfuse 

 directly through the walls of the blood-vessels into 

 the cavity of the nephridium, instead of transfusing 

 first into the body-cavity The ciliated funnel in 

 such a case may become almost (though not entirely) 



