398 TEXTBOOK OF ZOOLOGY 



nephridia extend through the wall to the exterior. The excreted 

 wastes accumulate in the eoelomic cavity and are moved into the 

 nephridia through the ciliated funnellike internal end, known as the 

 nephrostome. This eoelomic 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 eoelomic epithelium. The echino- 

 derms make use of direct diffusion as well as intracellular excretion 

 by which excreted materials are taken up from the eoelomic cavity 

 by the numerous phagocytic, amoeboid cells of the eoelomic 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 

 modification and condensation from segmentally arranged nephridial 

 tubules. The fact that in vertebrate embryos as well as in lower 

 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 to the exterior of the body are the 

 Malpighian corpuscles, each made up of a glomerulus and a Bow- 

 man* s capsule, and the coiled uriniferous tubules which discharge 

 the excretion through collecting tubules into the ureter at the pelvis 

 of the kidney. This canal leads to the cloaca in most vertebrates 

 below mammals (excepting some fish), or to a urinary bladder in the 

 mammals. 



