EEPOET ON THE PTEEOPODA. 
43 
The pedal ganglia are the largest of all, and somewhat triangular in form. A large 
number of nerves arise from them — seven in Pneumonoclerma (PL IV. fig. 9) — two 
anterior, two lateral, and three posterior. 
1. The more median of the anterior nerves (i) passes forward and innervates the 
foot. 
2. From the ventral surface of the ganglion, near the anterior margin, arises the large 
nerve (n) which passes to and ramifies in the fin. 
3 and 4. The two lateral nerves (hi and iv), of which the anterior is the stronger, pass 
forward to innervate the parts situated between the foot and the integuments of 
the head. 
5, 6, and 7. The three posterior nerves (v, vi, and vii) proceed to the part of the 
envelop of the body situated dorsally to the foot, behind the head, and in front 
of the visceral sac. These nerves are also found with the same distribution in 
the other Gymnosomata, and are incontestably homologous with those which 
Lacaze Duthiers has described under the name “ nerfs cervicaux.” 1 
The outermost of these nerves (v) anastomoses with the nerve (j) which 
springs from the pleural ganglion. Elsewhere I have only observed this anasto- 
mosis in Aplysia where it was not noticed by Lacaze Duthiers. 2 I have found it 
in all the typical Gymnosomata, in which it has not hitherto been seen except in 
Pneumonoderma by Souleyet. 3 
The plexus formed by these two nerves contributes to the innervation of the 
so-called “ cervical ” region. The details of its composition may vary according 
to the genus and even according to the species ; its general disposition is, 
however, always the same. 
To the posterior border of the pedal ganglion, near the origin of the middle 
cervical (vi) and near the point where the pedal ganglion is approximated to the 
pleural ganglion, is situated the otocyst (PI. V. fig. 3, h), which is just in 
contact with the pleural ganglion ; as has been mentioned above, its nerve (i) is 
derived from the cerebral ganglion. 
III. The pleural ganglion (c) is the smallest of those which make up the central 
nervous system, and is ovoid in form. It gives origin laterally to the nerve (PI. IV. fig. 
9 ,j) which unites with the outermost of the cervico-pedal nerves (v) in order to form the 
cervical plexus above described. The law of Lacaze Duthiers, according to which the 
pleural ganglion never gives off nerves, 4 must therefore be restricted to the aquatic 
Pulmonata. 
1 Du systeme nerveux des Gasteropodes pulmonis aquatiques, Arch. d. Zool. Exper., ser. 1, t. i. p. 493, 6 , 
2 Systeme nerveux des Gasteropodes (type Aplysie), Comptes rendus, t. cv. pp. 978-982. 
3 Voyage de la Bonite, Zoologie, Mollusques, pi. xv. fig. 37. 
4 Du systeme nerveux des Gasteropodes pulmones aquatiques, loc. cit., p. 494, 12°. 
