WATSON : SUB-CEREBRAL COMMISSURiE IN ORTHURETHRA. 281 



below it. The anterior end of the alimentary canal is thus surrounded 

 by a cerebral nerve-ring, which in some of the more archaic genera 

 recalls that found in the Amphineura. But the lower commissure is 

 usually narrower than the upper one, and it tends to disappear in 

 the higher Streptoneura ; in fact, in the Peotinibranchia it is only 

 known to occur in a very few of the most primitive members of the 

 group. 



In the Opisthobranchia it has been found that the cerebral ganglia 

 are united by both upper and lower commissures, not only in 

 primitive forms like Actceon, as shown by Bouvier, but also in 

 numerous other genera belonging both to the Tectibranchia and to 

 the Nudibranchia, as shown by Vayssiere, Pelseneer, and others. 

 But while the upper commissure, known simply as the cerebral 

 commissure, is short and broad, the lower commissure, which is 

 termed the sub-cerebral commissure, is long and very slender.^ 



Turning now to the Pulmonata, we find that, while the stout 

 cerebral comniissure is one of the most conspicuous parts of the 

 nervous system, very few writers have been able to discover a sub- 

 cerebral commissure, and some appear to doubt whether this lower 

 commissure is ever present in the Stylommatophora (excluding the 

 Ditremata). Fifty years ago, however, de Lacaze-Duthiers described 

 what is probably the sub-cerebral commissure in Limncea,^ and later 

 Amaudrut stated that it was present in Achatina panthera (Fer.), 

 Bulimus funki (Nyst.), Nunina cambodjiensis (Reeve), and Helix 

 aspersa, MlilL, four members of the Stylommatophora, which, it will 

 be noticed, not only come from four different continents, but belong 

 to four different families.^ In 1893 Plate showed that a sub-cerebral 

 commissure occurs in the Onchidiidse,* a fact subsequently confirmed 

 by von Wissel and Stantschinsky ; but it was not until 1917 that 

 Kunze ^ and Bang ® were able to announce the discovery of this 

 slender commissure in Helix pomatia, Lin., notwithstanding the 

 amount of work that had already been done on the anatomy of this 

 well-known species. I have myself been able not only to confirm 

 the existence of a sub-cerebral commissure in Helix aspersa and in the 

 Onchidiidae, but also to report its presence in more than one- species 

 of Apera,'' in Natalina queJcettiana (M. & P.),^ in Helicarion 



^ See, for example, Vayssiere, MoUusques de la France, vol. i, 1913, pi. xvi, 

 where is figured the central nervous system of four representative Opistho- 

 branchs. 



2 Arch, de Zool. Exper. et Gener., vol. 1, 1872, p. 453, pi. xvii, figs. 3, 4 ; 

 pi. xviii, fig. 8. 



'^ Bull. Soc. Philom. de Paris, ser. vir, vol. x, 1885, p. 107 ; Ann. Nat. Sci., 

 Zool., ser. vm, vol. vii, 1898, p. 127. 



* Zool. Jahrb. {Anat. u. Ontog.), vol. vii, p. 150, pi. xii, fig. 85. 



^ Zool. Anz., vol. xlviii, p. 234. 



« Ibid., p. 284, and fig. 1 (p. 282). 



7 Ann. Natal Mtis., vol. iii, 1915, p. 137, fig. 2 (p. 152), pi. xv. figs. 73, 74. 



8 Ibid., p. 138. 



VOL. XV.— OCT., 1928. 19 



