202 NERVOUS SYSTEM GASTEROPODA 



system varies greatly in different groups, ranging us it does I'rom 

 a condition of extreme complexity, in which the ganglia are 

 numerous and the commissures equally so, to that of considerable 

 simplicity, in which ganglia are almost entirely absent. 



The most important ganglia are (1) the cerebral^ which are 

 always placed above or on either side of the mouth, and from 

 which proceed the nerves of the eyes and tentacles ; (2) the pedal, 

 which in Gasteropoda are situated below the oesophagus, in 

 Pelecypoda at the base of the foot, and from which the nerves of 

 the foot and sometimes the acoustic nerve arise: (3) the pleural,^ 

 whose position varies considerably, but is always below the 

 oesophagus and slightly above the pedal ganglia ; these innervate 

 the mantle, branchiae, heart, and viscera generally. 



Gasteropoda. — The simplest form of nerve system as thus 

 understood occurs in the Ampliineura, and more particularly in 

 the Chitons. Here we find four longitudinal nerve-cords, parallel 

 to one another for nearly the whole length of the mollusc. The 

 two exterior cords probably represent tlie pleural, the two interior 

 the pedal nervous system. There being no head or tentacles, but 

 simply a mouth at the anterior end, the cerebral ganglia do not 

 exist, but they are represented by the curved ring formed by the 

 massing together of the two nerve-cords on each side. The only 

 distinct ganglia" are a pair of buccal ganglia (which are developed 

 on a pair of connnissures which pass forward from the cerebral 

 mass and innervate the lips and buccal region), and a much 

 smaller group, the sublingual. The two pedal cords are united by 

 a number of transverse parallel connectives, which recall similar 

 modes of connection in the Chaetopod worms and in Arthropocla. 



This quadruple set of nerve-cords is characteristic of all the 

 Amphineura, b\it the absence of ganglia is most marked in the 

 Chitons. In Froneomenia and JSfeomenia there is a distinct 

 cerebral ganglion, formed by the massing of the two ganglia into 

 one, while in Froneomenia the lateral cords are joined to the 

 pedal, as well as the pedal to one another, by connectives. In 

 Chaetoderma the cerebral ganglia, though adjacent, are distinct, 

 and both the pedal and lateral cords connect directly with them, 

 while there are no transverse connectives. 



^ Also known as lahial and supra-oesophageal ganglia. 



- Wiven, however {K. Sv. Vet. Ah. Handl. xxiv. 1892, No. 12), desciibcs trans- 

 verse connectives in Chaetoderma. 



