54 
THE VOYAGE OF H.M.S. CHALLENGER. 
2. A median nerve ( e ) innervating the buccal appendage (which Souleyet regarded as 
a gill, and to which he attributed a visceral innervation *) and the anterior tentacle. 2 
Each cerebral ganglion is connected to two infraoesophageal ganglia. The more 
anterior of the two is united with it by a rather short connective, which is easily 
distinguished by its transparence from the opaque white ganglia ; the posterior ganglion 
is almost in contact with the cerebral ganglion. 
The anterior subcesophageal ganglion (PI. Y. fig. 10, b) is the pedal ganglion. It is 
connected with its fellow by a commissure as short as that which connects the cerebral 
ganglia. In addition to this principal commissure, which does not appear in Souleyet’s 
figure, there is a second very slender one (PI. Y. figs. 8, 9, f) analogous to the 
second pedal commissure observed in the preceding families. 
Each pedal ganglion gives origin to three nerves (PI. Y. fig. 8, l, m), which supply 
the foot and the fins. On the posterior margin of the pedal ganglia are situated the 
otocysts (i). 
The smaller of the two subcesophageal ganglia (c) which are connected to either 
cerebral ganglia is connected posteriorly with a large azygous median suboesophageal 
ganglion. It follows hence that this little ganglion must be either the pleural ganglion 
or perhaps a ganglion of the visceral commissure. 3 
It cannot, however, be the pleural ganglion, for the pedal ganglion of the same 
side is not united to it by a connective ; on the contrary it is quite separated from 
it (see PL Y. figs. 8, 10); it is, therefore, a ganglion of the visceral commissure, as 
is shown also by the nerves which arise from it and innervate the visceral envelope. 
The azygous median ganglion ( d ) is elongated transversely and is larger than the 
two lateral ganglia ( c and e ). The two nerves which spring from it supply the viscera. 
The buccal ganglia (PI. Y. fig. 9, c), which were not observed by Souleyet, are 
situated under the oesophagus. The cerebro-buccal connective (e) is rather strong, and 
arises from the posterior margin of the cerebral ganglion on its oesophageal surface. The 
two buccal ganglia are close together, and situated between the oesophagus and the pedal 
ganglia. 
The arrangement of the nervous system of Halopsyche, as I have described and 
figured it, differs in some particulars from Souleyet’s account, especially as regards the 
constitution of the visceral commissure. 
In his description Souleyet supplied “ par l’analogie ” those details which he could 
1 Voyage de la Bonite, Zoologie, t. ii. p. 250. 
2 Compare the disposition of the buccal appendage and the anterior tentacle in the systematic Eeport on the 
Gymnosomata, Zool. Chall. Exp., part lviii. p. 53, fig. 4. 
3 I differ from Lacaze Duthiers in not regarding the pleural ganglia as a part of the visceral commissure. The 
distinctive character of the components of this commissure in the asymmetrical Mollusca is that they are devoid of 
symmetry, that is to say, they are individually unpaired even when the visceral commissure is paired as regards the 
number of its ganglia, or they are asymmetrical even when they are in pairs. The pleural ganglia, on the other hand, 
are always in pairs and equal to each other. 
