NERVOUS SYSTEM. INVERTEBRATA. 57 



portion, nerves are given off to the integument and S<>M-M>- 

 organs of the head. The posterior (pleuro-visceral) part 

 innervates the lateral and dorsal regions of the body- wall. 

 The two outer centres represent the pedal ganglia ; they 

 are in contact with the lateral surfaces of the pleuro-visceral 

 centres ; each sends several large nerves to the foot. As 

 in Archidorls the lateral parts of the supra-oesophageal mass 

 are united below the gut by a triple commissure enclosed 

 in a common neurilemma-sheath. Preserved in Goadby 

 solution. 



v. Ihering, Anat. des Nervensystemes . . . der Mollusken, 

 1877, p. 174. 



CEPHALOPODA. 



Note. In describing the Cephalopod nervous system it has been assumed 

 for convenience' sake that the funnel and mantle-cavity are situated on the 

 ventral aspect of the animal, and the beak at the anterior end. 



D. 59. The anterior parts of a male Pearly Nautilus (Nautilus 

 jtompilius) from which the viscera and left half of the 

 body- walls and funnel have been removed to show the 

 nervous system in situ. The central system shows no 

 differentiation into separate ganglia, but is coated evenly 

 in all parts by a continuous layer of ganglion-cells ; it is 

 situated entirely within the head region, supported by the 

 upper part of the large cephalic cartilages. Above the 

 oesophagus lies a transverse cylindrical bar of nervous tissue 

 directly continuous at either end with an antero-posteriorly 

 flattened optic ganglion. The extremities of the supra- 

 cesophageal bar are connected below the oesophagus by two 

 semicircular nervous bands that rise by a common origin 

 from either end of the bar and slant respectively for- 

 wards and backwards towards the ventral surface of the 

 oesophagus. The anterior semicircular band is thick 

 at either end, but rapidly tapers towards its middle, 

 until beneath the oesophagus it forms a narrow com- 

 missural strand. It gives off on either side from the 

 lower end of its thickened region a stout nerve for the 

 funnel, and from its anterior surface, between this point 

 and its junction with the supra-oesophageal bar, a large 

 number of nerves for the outer and lateral series of tentacles. 



