72 
THE VOYAGE OF H.M.S. CHALLENGER. 
more important by reason of its volume than the thin supraoesophageal thread mentioned 
by Dietl in Eledone alone. 
I will also add that probably in all the Opisthobranchia (Bullidse , 1 Umbrellidae , 2 
Pleurobranchidse , 3 Aplysiidae , 4 many Nudibranchia , 5 &c.), as well as in the Cymno- 
somatous Pteropoda , 6 there is an infraoesophageal cerebral commissure, which von Jhering 
has called subcerebral. It is much more slender than the supraoesophageal cerebral com- 
missure, but no one has ventured to suggest in consequence of this, that in the Opistho- 
branchia the cerebral ganglia were primitively suboesophageal. 
So far as l can see, the supraoesophageal brachial commissure of Eledone is of no more 
morphological value than the suboesophageal cerebral commissure of the Gastropods just 
mentioned. 
Nothing is further from complete demonstration than the hypothesis according to 
which the brachial ganglia are cerebral in origin. On the other hand, many proofs show 
that they are only a segmented part of the pedal ganglia. 
Such transverse segmentations of ganglia are not rare among the Mollusca. In addi- 
tion to the instance already quoted of the pedal ganglia of Natica and those of the 
Marseniidse , 7 we may mention the siphonal ganglia of Cyprxa , 8 the tentacular ganglion 
of Pleurobranchus , 9 the siphonal ganglion of many Pelecypoda, &c. Even in the Decapod 
Cephalopoda, too, there is an instance of the division of the cerebral ganglia, quite com- 
parable to that of the pedal ganglia which has led to the formation of the brachial ganglia. 
Cheron 10 has shown, and his statement has not been disputed, that the ganglion known 
as the “ superior buccal,” and still called by that name by Stieda 11 and Bobretzky , 12 is 
nothing else than the anterior part of the cerebral ganglia. I am able to state further 
that in the embryos of Sepia the formation of these “ buccal ” ganglia and their separation 
from the cerebral ganglia takes place in a manner quite parallel to that which has been 
advanced above as regards the formation of the brachial from the pedal ganglia. 
It might be objected that in Ommatostrephes, for example , 13 the brachial ganglia are 
1 Vayssiere, Recherches anatomiques sur la famille des Bullides, Ann. d. Sci. Nat., Zoologie, ser. 6, t. ix. pi. vi. fig. 48 
( Gastropteron ), pi. viii. fig. 69 ( Doridium ), pi. ix. fig. 81 ( Philine ), pi. xi. fig. 101 ( Scaphander ), pi. xii. fig. 114 {Bulla). 
2 Vayssiere, Recherches zoologiques et anatomiques sur les Mollusques Opistobranches du Golfe de Marseille, 
i. Tecti branches, Ann. Mus. Marseille, t. ii. pi. vi. fig. 149. 
3 Vayssiere. ibid., p. 144. 4 Vayssiere, ibid., pi. iv. fig. 94. 
6 Von Jhering, Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, p. 283. 
6 Wagner, Die Wirbellosen des weissen Meeres, Bd. i. pi. xii. fig. 1. 
7 Bergh, Die Marseniaden, Zool. Jahrbiicher, Bd. i. p. 168, fig. 1. 
8 Von Jhering, loc. cit., pi. viii. fig. 35. 
0 Von Jhering, ibid., pi. xi. fig. 8. 
10 Recherches pour servir a l’histoire du syst&me nerveux des Cephalopodes dibranchiaux, Ann. d. Sci. Nat., Zoologie, 
s6r. 5, t. v. 
11 Studien uber den Bau der Cephalopoden, Zeitschr. f. wiss. Zool., Bd. xxiv. 
12 Observations on the development of the Cephalopods, Proc. Soc. Friends of Nat. Hist. Anthrop. and Ethnogr. Moscow, 
1876 (Russian). 
13 On the Nervous System of Ommastrephes todarus, Ann. and Mag. Nat. Hist., ser. 2, vol. x. pis. i., ii. 
