MORPHOLOGY OF ANIMALS 



by the general name of nephridium. The forms of 

 nephridia differ considerably in different phyla, but two 

 principal types may be recognized; these are the proto- 

 nephridia, or water-vascular system, and the meta- 

 nephridia. 



Proto-riephridia are found in flat-worms (Fig. ii. A) 

 and rotifers; that is, among worm-like animals without a 

 true coelom; they are also found as the larval excretory 

 organ ("head kidneys") in annelids. They consist of more 

 or less branched tubules opening at one or more places 

 to the exterior, while the inner end of each tubule is closed 

 by a single large cell which bears a tuft of long cilia pro- 

 jecting into the lumen of the tubule (Fig. 19, A). This 

 tuft beats with undulatory movement and looks somewhat 

 like the flickering flame of a candle, whence it is called a 

 ''flame' and the large cell which bears it is a ''flame cell.'" 

 The tubule itself is usually composed of a single series of 

 long glandular cells ("drain pipe" cells), with the lumen 

 running through the middle of each cell. In larger branches 

 of the protonephridium the walls of the tubule may be 

 formed of many cells which are ciliated on the side next 

 the lumen. These cilia as well as the "flame" drive fluids 

 within the lumen to the exterior. It is probable that these 

 fluids are transuded body fluids containing nitrogenous 

 waste substances which first appear as vacuoles in the 

 flame cells and then discharge into the lumen of the tubule 

 (Fig. 19, A) 



Meta-nephridia are found among annelids, moUusks, 

 molluscoids, prototracheates, and chordates, while a modi- 

 fled form exists in crustaceans. Typically each meta- 



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