NERVOUS SYSTEM. INVERTEBRATA. 47 



of the pedal cord. From these swellings, which much 

 resemble the labial lobes of Haliotis, two pairs of con- 

 nectives are given off one to the ganglia of the subradular 

 organ (imperfect in the specimen), the other to the buccal 

 ganglia, which lie as usual upon the posterior surface of 

 the buccal mass, between the oesophagus and radular sac. 

 The buccal system in this and certain other species forms a 

 second complete ring around the gut : the buccal ganglia 

 being united by commissures both below the oesophagus 

 and above the roof of the buccal mass. The buccal ganglia 

 probably innervate the whole alimentary canal. 



0.0. 1305 c. 

 Burne, Proc. Make. Soc., vol. ii. 1896, p. 4. 



D. 45. The anterior part of the nervous system of the same 

 species of Chiton, isolated to show the two nervous rings 

 (circumoral and buccal) that surround the alimentary 

 canal. 0. 0. 1305 D. 



GASTKOPODA. 

 Bouvier, Ann. Sci. Nat., ser. 7, t. iii. 1887, p. 1. 



D. 46. An isolated specimen of the nervous system of an Ormer 

 (Haliotis tuberculata) . The cerebral ganglia are small but 

 fairly distinct, and are united in front of the mouth by a 

 long ribbon-like commissure. From the lateral parts of each, 

 two connectives pass beside the buccal mass to a compound 

 pleuro-pedal ganglion beneath the gut. The greater part 

 of this ganglion belongs to the pedal system, and extends 

 backwards in the substance of the foot as a pair of flattened 

 cords, fused to one another and to the pleural centres at their 

 anterior end, and united at intervals by nine transverse 

 commissures. Nerves are given off from this " ladder-like " 

 pedal system to the foot and epipodium. The pleural 

 ganglia form a pair of indefinite excrescences on the dorsal 

 surface of the fused anterior ends of the pedal cords. 

 They give off a pair of large mantle-nerves, and are united 

 together, as in other Prosobranchs, by a commissural 

 loop (visceral loop), that in its passage from ganglion to 

 ganglion is twisted in the form of an 8. This twisted or 

 streptoneurous loop is characteristic of the Prosobranchs, 



