NERVOUS SYSTEM 



139 



Smallwood (1926, 1930), Prosser (1934a), Hanstrom (1928), Hess 

 (1925a), Langdon (1895) and Stephenson (1930), but an outline 

 may be of use in pinpointing the structures mentioned later. 



The main nerve trunk of the body runs ventrally from segment 

 4 to the rear end, and has one ganglion, a collection of nerve cells, 

 to each body segment. Anteriorly the cerebral ganglion is a bilobed 

 structure found dorsally in segment 3, giving rise to two stout 

 nerves that pass forward to the prostomium which is well supplied 

 with sense organs (Langdon, 1895). The nerves supplying the two 

 most anterior segments arise from the circum-oesophageal com- 

 missures which envelop the oesophagus in segment 3. Segment 4 



Fig. 41 . Anterior nervous system of Lumbricus terrestris. I-VI 

 segments 1-6 (from Hess, 1925). 



contains the sub-oesophageal ganglion which represents the fused 

 ganglia of segments 3 and 4, and the innervation of both segments 

 stems from this ganglion. In a typical body segment three pairs of 

 nerve trunks originate from the ventral nerve cord and pass out- 

 wards to the body wall. The anterior pair arise just behind the 

 anterior septum of the segment anterior to the ganglion and at a 

 distance from the other two pairs w^hich stem from the segment 

 ganglion and are close together. The third nerve on each side gives 

 off a small branch to the septum at the posterior side of the seg- 

 ment. In the caudal segment, which apparently is the equivalent of 

 two body segments, five to six nerves are found (Hess, 1925a) 

 (Fig. 41). 



