III.] THE EARTHWORM. 263 



the pharynx {visceral 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 enlargements formed upon 

 the latter, 



c. The ganglionic nerves ; two pairs for each somite. 

 They arise from the ganglionic swellings — trace 

 them to the body wall. 



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 a. Note that each bears a well-marked 

 sheath and a coagulable contents. (Cf. p. 246.) 



y. The cut ends of the sub- and lateral neural- blood- 

 vessels, together with their anastomoses, all of 

 which are buried up in a. 



8. 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 



