148 BIOLOGY: GENERAL AND MEDICAL 



through an external opening or pore. The blood in the 

 vessels circulates through a vascular plexus about the 

 convoluted portion of the tubule permitting its cells to 

 take up the offensive substances and transmit them to 

 the fluid constantly passing through the lumen. Here 

 we have a most important and interesting specialization 

 and differentiation of cellular activity, the sole function 

 of these complicated organs being the absorption of 

 waste products from the blood and their elimination 

 from the animal. 



This plan of having a tubular gland whose epithelial 

 cells secrete the solids which are carried out by a passing 

 current of water finds no improvement as the scale of 

 zoological life is ascended. There are various modifica- 

 tions seen among special groups of animals, as among the 

 Crustacea, where special excretory glands are situated 

 near the mouth parts and are developed upon a different 

 principle, but in the main the only difference between 

 the nephridium of the annelid and the kidney of the ver- 

 tebrate is to be found in the number of component ele- 

 ments and the exact means by which the watery part of 

 the excretion is provided. 



The chief excretory organs of the vertebrates are 

 known as kidneys, which upon superficial examination 

 appear highly complex, though upon investigation are 

 easily resolvable into a combination of units each of which 

 is a tubular structure whose epithelial cells secrete the 

 solids which are washed away by the water supplied by a 

 capillary tuft at its commencement. Thus each struc- 

 tural unit in the mammalian kidney is the homologue of 

 the nephridium of the worm. 



Innervation and Coordination. As structural differ- 

 entiation and specialization increase, and that cellular in- 

 dependence, by virtue of which any cell of the primitive 

 composits seems able to perform any function, gives place 

 to organized structure in which certain cells are set aside 

 for the performance of particular functions, means of 

 communication between the different cell groups becomes 



