188 HUGH WATSON. 



(figs. 73 and 74). It thus forms the most anterior element of 

 the nerve-collar. Amaudrut ^ has described a similar sub- 

 cerebral commissure in Achatina panthera (Fer.), Buli- 

 mus funki {Nyst.), Nanina cambodjiensis [Reeve), and 

 Helix aspersa Mull.; but hitherto this slender commissure 

 has been noticed in only a very few Pulmonates and in none of 

 the carnivorous forms. I have found it, however, in Natalina 

 quekettiana [M. Sr P.), the only member of the Rhy tididae 

 whose ganglia I have examined microscopically ; and I 

 believe that if malacologists would examine the central 

 nervous system of snails with greater care they would find it 

 in many other pulmonate genera. For among the marine 

 Euthyneura a sub-cerebral commissure has been observed 

 not only in the Pleurobranchid^ and in numerous Nudi- 

 branchs,- but also in so primitive a form as Actfeon.'^ 

 Moreover, I am inclined to regard the sub-cerebral com- 

 missure as homologous with the important labial commissure 

 found in the Aspidobranchia, and in Vivipara and 

 Ampullaria among the Pectinibranchia, as well as in 

 the Amphineura, S cap hoped a, and Cephalopoda."* In 

 most of these forms the cerebro-buccal connectives arise 

 from the labial commissure instead of from the cerebral 

 ganglia themselves, and in Apera the cerebro-buccal con- 

 nectives arise from the ganglia very close to the ends of the 

 sub-cerebral commissure. 



A little further back arise the thick cerebro-pedal con- 

 nectives ; and behind this again, nearly at the posterior end 

 of the ganglia, the almost equally broad cerebro-pleural con- 

 nectives arise. In the specimen of Apera dimidia that I 



' 'Bull. Soc. Philom. Paris' (7th ser.}. 1885-8(3. vol. x. pp. 107-117: 

 'Aim. Nat. Sci., Zool.,' 1898, vol. vii, p. 127. 



- Pelseneer, P., ' Mem. Couronne Acad. Roy. Belg.." 1893. ex. vol. liii. 

 p. 69. 



3 Bouvier. E. L.. ' Bull. Soc. Philom. Paris " (8th ser.), 1893, vol. v, p. 67. 



'' Pelseneer has shown that the so-called " labial commissiu-e " which 

 Pleurobranchsea and a few other forms possess in addition to the 

 sub-cerebral commissure, is merely an anastomosis of two of the nerves 

 to the lijDS (op. cit.. p. 33). 



