NEPHEIDIA 



379 



into a secretion shall pass into some kind of a duct through 

 which it escapes to the outside. The function of the kid- 

 ney of man is the removal of nitrogenous wastes, that is, 

 the removal of the wastes resulting from metabolism of 

 proteid in the protoplasm. We have already discussed 

 the adaptations of the lower animals for the removal of 

 carbon dioxide under the head of the respiration. It re- 

 mains to examine the nitrogen waste removers of these 

 lower forms, and in the following paragraph we shall con- 

 sider only the organs which perform the same action as 

 the human kidneys. 



Nephridia. — One of the simplest forms of kidney is found 

 in the group of mollusks and earthworms. It is called a 

 nephridium, and as the earth- 

 worm is perhaps the most fa- 

 miUar type Ave shall describe the 

 organs as seen in this animal. 

 If we tiu-n the earthworm over 

 and examine its ventral surface 

 we can make out a double row 

 of pores, two to a seginent, ex- 

 tending from the head end to 

 the tail end. If we dissect the 

 worm and trace the internal 

 connections of these pores we 

 find them to be the mouths of 

 long coiled ducts which, after 

 twisting about in the body 

 cavity, end in funnel-shaped, ciliated mouths in the seg- 

 ment just in front of the segment in which the pore is 

 located. (See Fig. 176). Further examination shows this 

 duct to be formed in part by secreting cells and surrounded 



Fig. 175 — A nephridium; ,S, sep- 

 tum; ^, funnel; O, pore or ex- 

 ternal opening. 



