III.] THE EARTHWORM. 26^ 



the pharynx {znsceral nerves). On reaching the 

 ventral surface the commissures become firmly 

 united and pass on, to the extreme end of the 

 body, as longitudinal ventral commissures. Note 

 the metameric gangliform e7ilargements formed upon 

 the latter, 



c. The ganglionic nen^es ; two pairs for each somite. 

 They arise from the ganglionic swellings — trace 

 them to the body \vall. 



d. The interganglionic nerves ; one pair for each somite, 

 arising from the longitudinal commissures at the 

 bases of the mesenteric septa, to which they are 

 largely distributed. 



e. The histology of the nervous axis and its associated 

 structures (for method of preparation see Sect. H.). 



Examine transverse sections under a high power, 

 note — ■ 



a. The sheath of the nervous system ; a deeply 

 staining investment, mainly composed of the cut 

 ends of muscular fibres. 



/?. The neurochord. It consists of three transparent 

 thick-walled " tubular-fibres " (a larger central 

 and two smaller lateral ones) buried up in the 

 roof of rt". 



y. The cut ends of the sub- a?id lateral iieural blood- 

 vessels^ together with their anastomoses, all of 

 which are buried up in a. 



B. The ventral nervous axis. Note the close appo- 

 sition of the two commissures. Each is com- 

 posed of a transparent matrix in which are seen 



