250 LECTURE XII. 



of nerve's are given off from each ganglion, and a third pair comes off 

 from the intermediate chords. The terminal or anal ganglion distri- 

 butes a plexus of nerves to that termination of the body. 



In the Nereis the abdominal ganglions are more distinctly bilobed 

 than in the earth-worm, and the super-oesophageal ganglion is rela- 

 tively larger, having to furnish nerves to both antennee and ocelli ; the 

 latter projects as a black spot from the base of each of the two chief 

 cephalic tentacles. The pairs of ganglions developed upon the ventral 

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

 very close together. In the Eunice gigantea there are upwards of 

 1000 ganglia ; but this complicated condition of the nervous system 

 is more apparent than real, and, like the multiplication of the pul- 

 satile sinuses of the vascular system in the same animal, depends 

 upon the vegetative repetition of like parts, without any mutual sub- 

 ordination in reference to the performance of a special office. 



In the Aphrodita the body is broader and thicker than in other 

 Anellids, and begins to exhibit that concentration which characterises 

 its form in the higher Articulata. But the segmental nervous ganglions, 

 though more closely approximated, are yet not confluent at any central 

 part. The brain is heart-shaped, having its bilobed base turned back- 

 wards, and connected in the usual manner by large oesophageal columns 

 with the inferior ganglion. The antennal nerves are continued from 

 the apex. The visceral nerves are given off from the oesophageal 

 circle, and pass to the upper surface of the intestine, and there swell 

 into a small ganglion. The sub-oesophageal ganglion is of large size, 

 and bifurcates anteriorly : the second ganglion is situated close by 

 the first, and gives off two pairs of nerves: the third to the fifteenth 

 ganglions send off respectively three pairs of nerves, the first of 

 which corresponds with the interganglionic nerve in the earth-worm, 

 and supplies the branchial organs ; the second pair is distributed to 

 the ventral muscles ; the third, to the lateral and dorsal muscles. The 

 abdominal ganglions, which succeed the fifteenth, send off each two 

 pairs of nerves, and gradually diminish and approximate at the pos- 

 terior extremity of the body. In this highly organised anellid the 

 nerves may be distributed into those of special sense (antennal), the 

 sentio-volitional, the excito-motory, the sympathetic or stomato- 

 gastric, and the respiratory. The leech is very susceptible of meteoric 

 changes ; and instruments have been constructed, of which a very 

 ingenious one was shown in the '* Great Exhibition of 1851," in order 

 to make the characteristic movements of these Anellids subservient 

 to the purposes of a barometer. 



" Those who have watched the habits of the Nereids will scarcely 

 doubt that they are gifted with the power of discriminating external 



