PHYSIOLOGY 189 



lated excretion down the tubule. The waste liquid of the surround- 

 ing tissues diffuses into this cell. The main excretory ducts open to 

 the surface of the body by excretory pores. This arrangement is 

 sometimes called a protonephridial system. 



The nephridial system is found in Annelida and has been studied 

 in connection with the earthworm. Here a coelomic cavity is present, 

 and a series of segmentally arranged pairs of coiled tubes or 

 nephridia extend through the wall to the exterior. The excreted 

 wastes accumulate in the coelomic cavity and are moved into the 

 nephridia through the ciliated funnellike internal end, kno^^^l as the 

 nephrostome. This coelomic fluid is drawn into the canal of the 

 nephridium by the beating of the cilia and is delivered to the outside 

 of the body at the nephridiopore of the next segment. 



The green glands of crayfish are much more concentrated, although 

 they are modified nephridia. They function as a pair of unit organs, 

 each opening by a duct on the basal segments of the antennae. In 

 mollusks there are both nephridia, known as pericardial glands, and 

 the special cells formed from the coelomic epithelium. The echino- 

 derms make use of direct diffusion as well as intracelhdar excretion 

 by which excreted materials are taken up from the coelomic cavity 

 by the numerous phagocytic, amoeboid cells of the coelomic fluid. 

 These cells wander out into the cavities of the respiratory organs 

 where they coalesce into large masses, and finally with their enclosed 

 granules are cast out through the membranes of the respiratory 

 papillae. Soluble materials in solution also diffuse through the mem- 

 branous walls of these structures. In the insects excretion is taken 

 care of by the Malpighian tubules, which are considered modified 

 nephridia. They are bunched in the posterior part of the body cavity 

 and discharge excretions into the intestine at its junction with the 

 rectum. 



Kidneys 



The chief excretory organs of vertebrates are called kidneys, and 

 they are thought by some authors to have developed by modifica- 

 tion and condensation from segmentally arranged nephridial tubules. 

 The fact that in vertebrate embryos as well as in low^er chordates, even 

 the frog, these tubules open into the coelom as nephrostomes, makes 

 it seem possible that in vertebrates as well as in annelids the coelom 

 was once important in excretion. The essential structures of the 

 kidney for taking waste substances from the blood and delivering it 



