III.] THE EARTHWORM. 267 



d. The follicle of the seta; a small granular proto- 

 plasmic mass internal to each pair of functional 

 setse. It lies close under the peritoneum, and 

 usually bears two conical immature setae. 



4. The wall of the alimentary canal. This will be found 

 to vary slightly in detail in different regions, — the in- 

 testine is here described. 



a. The epithelium, a single layer of elongated cells, 

 ciliated and lined internally by a delicate cuticular 

 membrane. 



b. The muscular layer — feebly developed. It consists 

 of an internal circular and an external longitudinal 

 series of fibres. 



The thickening of the wall of the gizzard is due to 

 an excessive development of the circular fibres. 



c. The sub-epithelial tissue ; a vascular layer interposed 

 between a and b. Look for the cut ends of its 

 small vessels. 



d. The yellow-brown tissue; a thick investment for the 

 whole, immediately external to b (outside the wall 

 of the whole viscus). Its component cells (chlora- 

 gogen cells) are elongated and club-shaped, each 

 containing a single oval nucleus and crowded with 

 minute highly refractive globules. 



Examine the muscular fibres of the mesenteric 

 septum. An aggregation of circular fibres is fre- 

 quently met with immediately external to the ali- 

 mentary canal. Many of its radial fibres can be 

 traced into the body-wall. 



5. The central nervous system, described in Sect. G. e. 



