NERVOUS SYSTEM. INVERTEBBATA. 13 



fibres are in connection with ganglion-cells that lie in pairs 

 on the ventral surface of a certain number of the posterior 

 ganglia of the chain. Anteriorly the lateral fibres are said 

 to break up in. the suboesophageal ganglion. The median 

 fibre apparently arises from cells in the same ganglion. 

 Branches from all three fibres have been seen to enter the 

 lateral nerves. It is probable that the giant fibres act as a 

 direct path of communication between all regions of the 

 nervous system, and are particularly concerned in bringing 

 about the simultaneous contraction of the whole body-wall, 

 such as takes place when the worm shoots back into its 

 burrow. In creeping, contraction occurs slowly segment 

 by segment. The co-ordination of this segmental contrac- 

 tion is apparently due, not so much to connections within 

 the central nervous system as to an orderly sequence 

 of independent stimuli, each of which is caused by the 

 stretching of the integument of any one segment by the 

 contraction of the longitudinal muscles of the segment 

 in front. 



Friedlander, Zeits. wiss. Zool., Bd. xlvii. 1888, p. 47, & 

 Bd. Iviii. 1894, p. H61 (Anat.). 



Friedlander, Arch. ges. Physiol., Bd. Iviii. 1894, p. 168 

 (PliyswL). 



D. 8. An Earthworm (Lumbricus terrestris) with the ventral 

 body-walls removed to show the nervous cords, their 

 ganglia and lateral branches. 0. C. 1296. Hunterian. 



HIRUDINEA. 



D. 9. The ventral body-walls of a Leech (Hirudo medicinalis) 

 with the nervous system exposed from the dorsal aspect. 

 The central system lies amongst the parenchyma internal 

 to the body-walls. The cerebral ganglion is a small bilobed 

 body situated close behind the jaws on the upper surface of 

 the pharynx ; it is of very simple construction and probably, 

 as its removal causes no appreciable difference in the actions 

 of the animal, differs little if at all in function from the 

 ganglia of the ventral chain. It innervates the cephalic 

 sense-organs, and jaws. A pair of extremely short connec- 

 tives unite the cerebral ganglion around the anterior part of 



