STRUCTURE OF THE EARTHWORM. 



157 



III. The Setae. In a section which contains setae, no- 

 tice : 



a. The complicated system of muscles running from the 

 inner end of the seta to the surrounding integument. 



b. Small, partially developed setae near the inner end of 

 each large one. 



c. The sheath around the outer end of the seta, formed 

 by a tubular infolding of the cuticle. 



IV. The Nervous /System. 



h 



FIG. 90. 



FIG. 90. Transverse section through the ventral ganglia, near the 

 middle of the body. (From Claparede. Taf. xlvii., Fig. 4.) 



a. Surface epithelium, b. Muscular layer. c. "Tubular fibres." 

 d. The two ganglia. /. Outer layer with large ganglion cells, g. Lateral 

 nerves, h. Ventral blood-vessel. 



Examine with a high power, two hundred and fifty to 

 five hundred diameters, a section which passes through 

 one of the ganglionic enlargements of the ventral nerve 

 cord, and notice : 



a. The layer of epithelium (Fig. 90, a) which forms its 

 outer sheath. 



