NERVOUS SYSTEM. 211 



from which the two principal nervous chords extend back- 

 wards beneath the abdominal viscera in the gasteropods as 

 in the bivalved mollusca, as seen in the annexed figure of 1 he 

 nervous system of the aplysia fasciata (Fig. 90.) In the 

 aplysia, as in many of the higher forms of gasteropods there 

 is a large median supra-cesophageal ganglion (a,) from which 

 the organs of the senses receive their nerves, and from this 

 single superior ganglion (a,) proceed downwards and laterally 

 two nervous bands to connect it with the usual pair of late- 

 ral cesophageal ganglia (b, c.) Two nervous branches (/,) 

 likewise proceed downwards and forwards from the" cerebral 

 ganglion (,) to form a small anterior collar around the oeso- 

 phagus (i 9 ) and an inferior single ganglion (ff 9 ) is seen be- 

 neath the muscular bulb of the oesophagus, where these 

 branches meet. The posterior large nervous collar is com- 

 pleted by an inferior transverse band passing between the 

 lateral ganglia (b, c,) and beneath the oesophagus (i.) From 

 the lateral cesophageal ganglia (b, c,) two nervous trunks (d 9 d,) 

 extend backwards beneath the divisions of the stomach (k, I, 

 m, n 9 ) and along the ventral surface of the abdomen to near 

 the bulb of the aorta (t 9 ) where they meet with a single gan- 

 glion (e,) considered, from its position, its distribution and its 

 attachment to that arterial trunk, as a sympathetic ganglion. 

 The cerebral ganglion (#,) placed above the oesophagus, and 

 connected through the lateral ganglia (b, c,) with these two 

 longitudinal columns, has a quadrangular form, a reddish 

 brown colour, and is enclosed in a tough cranial membranous 

 capsule immediately above the posterior end of the bulb of 

 the oesophagus. The same reddish coloured nucleus and 

 granular structure are seen in all the ganglia of the aplysia, 

 and from the toughness of the neurilematous covering of the 

 ganglia and nerves they can be easily injected like vessels, as 

 in most other molluscous animals. The lateral ganglia (90. 

 b, c,} have each a trilobate form, and the nervous bands 

 which connect them with the brain have a distinct appear- 

 ance of separate component columns. There are two fila- 

 ments which proceed backwards from these lateral ganglia, 

 like the origins of the sympathetics of Crustacea, and which 

 here form by their union an arch around the trunk of the aorta. 

 The anterior bilobate sub-oesophageal ganglion (90. g 9 ) gives 

 off eight nerves to the oesophagus, the salivary glands and 



p 2 



