238 ANIMAL PHYSIOLOGY. [LESSON 71. 



sub-oesophageal ganglion (c). From this ganglion the muscles of 

 the three serrated jaws, as well as the principal muscles of the oral 

 (mouth) sucker, derive their nervous influence. Those who have 

 watched the vigorous workings of this part in a hungry leech, begin- 

 ning its feast, will not be surprised at the great development of the 

 nervous centre of the suctorial and maxillary mechanism. 



1082. Two chords, in such close apposition as to seem a single 

 nervous band, are continued from the sub-03sophageal ganglion (sub, 

 under, beneath) along the middle of the under part of the abdomen, 

 attached to the ventral (ventre, belly) integument, and enclosed, as it 

 were, by the great ventral vein. Twenty-one equidistant ganglions 

 are developed upon these chords, which distribute their filaments to 

 the adjoining segments by two diverging trunks on each side. The 

 segments indicated by the external circular indentations of the in- 

 tegument are much more numerous than the ganglions. A simple 

 nervous filament extending from the 03sophageal ganglion along the 

 dorsal aspect of the alimentary canal, has also been detected in the 

 leech, and this forms the first trace of a distinct sympathetic nerve 

 as yet seen in the animal kingdom. 



1083. In the earth-worm (angle-worm) the brain or supra-oeso- 

 phageal ganglion (supra, above) consists of two lateral lobes, which 

 send off small nerves to the proboscis, and the two large connecting 

 cords to the sub-o3sophageal ganglion. The two ventral nervous 

 trunks are more distinct from each other than in the leech, but the 

 ganglions are relatively smaller and more numerous, corresponding 

 in number with the segments of the body. 



1084. In the Nereis, a marine worm, some species of which grow 

 to the extent of two feet in length, the abdominal ganglions are more 

 distinctly bilobed than in the earth-worm, and the supra -oasophageal 

 ganglion is relatively larger, having to furnish nerves to both an- 

 tennae and ocelli. The pairs of ganglions developed upon the ven- 

 tral chord correspond with the segments of the body in number, and 

 are very close together. 



1085. In the species which attains the great length indicated, 

 Eunice gigantea, the nervous system consists of upwards of 1,000 

 ganglia. 



In the Epizoa, the nervous system of Actheres percarum (Fig. 

 1204) possesses a single cephalic ganglion, placed on the ventral sur- 

 face, from which are distributed two principal chords (g, g) } extend- 

 ing along the under surface of the body. 



