i66 GENERAL BIOLOGY 



extend outward on every side to the body wall, expand it, 

 making it an efficient organ for sucking the food into the 

 mouth. Behind the pharynx, the esophagus extends as a 

 slender passage-way leading to the thin walled and disten- 

 sible crop, which lies at about the fifteenth segment and this 

 immediately adjoins the thick walled muscular chitin-lined 

 gizzard, which comes nearest to a grinding organ of any- 

 thing possessed by the worm. The remaining part of the 

 alimentary canal is undifferentiated stomach-intestine. In 

 this the final digestion and absorption of the food occurs. 

 If the alimentary canal be slit open and washed out (it 

 will evert itself in a freshly killed worm by the contraction 

 of the circular muscles of its walls) a sudden change in 

 the color of the lining tissues will be seen at the beginning 

 of the stomach-intestine , the nature of which will be indi- 

 cated later when development is considered. 



Nephridia. — In the coelom at either side of the enteron 

 in every typical segment will be seen a delicate whitish 

 organ, at first appearing like a tangle of whitish threads; 

 it is the nephridium, a simple excretory organ, whose prin- 

 cipal function is the removal of nitrogen waste from the body. 

 It is a coiled and twisted tube, which opens into the body 

 cavity by a funnel-shaped aperture lined with cilia, and to 

 the outside by an excessively minute pore that is situated 

 near the lower line of setae. The body of each nephridium 

 lies in one segment, and the funnel aperture and short stalk- 

 like beginning of the tube (which perforates the septum low 

 down near the nerve cord) lies in the preceding segment. 

 The relations of the nephridia to the body as a whole, and 

 to the other internal organs, the principal trunks of the 

 circulatory system, the position of the setae and of the 

 principal muscle bands are indicated in the accompany- 

 ing diagram of a cross section of the worm (fig. 104a). 

 The muscle system consists of the broad outer sheet 



