202 NERVOUS SYSTEM — GASTEROPODA chap. 



nervous system varies greatly in different groups, ranging as it 

 does from a condition of extreme complexity, in which the ganglia 

 are numerous and the commissures equally so, to that of consid- 

 erable 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 

 l^edal^ 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 Amphineura, and more particularly in 

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

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

 two exterior cords probably represent the pleural, the two inte- 

 rior 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 commissures 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 connexion in the Chaetopod worms and in Arthropoda. 



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

 Amphineura, but the absence of ganglia is most marked in the 

 Chitons. In Proneomenia and Neomenia there is a distinct 

 cerebral ganglion, formed by the massing of the two ganglia 

 into one, while in Proneomenia 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 dis- 

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

 them, while there are no transverse connectives. 



1 Also known as lahinl and mpra-oesopharjeal ganglia. 



-WiA-en, however {K. Sv. Vet. Ak. Handl. xxiv. 1892, No. 12), describes 

 transverse connectives in Chaetoderma. 



