234 THE CEPHALOPHORA. $ 208.. 
The nervous centre of the genus Sagitta is in many respects like that 
of the Heteropoda. A hexagonal cerebral ganglion lies upon the upper 
surface of the cesophagus; another quite large is situated in the centre of 
the ventral surface of the trunk. These intercommunicate by two large, 
very long cords. The cerebral ganglion gives off two pairs of nerves; 
—of these the anterior are distributed to the base of the oral hooks, and 
the posterior to the organs of vision; each sends, internally, a filament 
which passes backwards and joins at the middle of the posterior part of the 
head with the one from the opposite side, thus forming a loop. The ven- 
tral ganglion sends off backwards two considerable, diverging nerves, from - 
whose external surface pass off numerous, delicate, cutaneous filaments. © 
2. With some Tectibranchia, a simple cerebral ganglion above, and 
two others, quite widely separated, below, are, all three, connected together 
by as many cords, thus forming a large oesophageal ring. 
3. With many Pteropoda the cerebral ganglia are wanting, while the 
lower portion is highly developed. This last is composed of two or three 
pairs of ganglia blended together, and from which passes off a simple 
commissure embracing the cesophagus. 
4. With very many of the Apneusta and Nudibranchia, as also with 
several other Heterobranchia, the cerebral mass is highly developed,. 
while the remaining part of the esophageal ring consists of a simple nerv- 
ous cord. The two or four cerebral ganglia are either connected by trans- 
verse commissures, or intimately blended together. 
p. 99, Tav. LXIII. (Carinaria). Pterotrachea 
has a similar disposition. According to Delle 
Chiaje (loc. cit. Tay. LXIII. fig. 14, Tav. LXIV. 
fig. 11), a short commissure arising from the cere- 
bral ganglionic mass, embraces, in a ring-like man- 
ner, the cesophagus of Carinaria and Pterotra- 
chea ; but this is not mentioned by either Cuvier, 
or Miine Edwards. | 
3 See Krohn, loc. cit. p. 12; fig. 2, 5, 13.* 
4 With Aplysia, according to Cuvier, Mém. loc. 
cit. p. 22, Pl. III. IV., and with Pleurobranchus, 
according to Delle Chiaje, Memor. loc. cit. Tav. 
XLI. fig. 8,0. v. v. I have found the esophageal 
ring arranged in the same way with Pleuro- 
branchaea. 
eyes and tentacles, has a transversal row of six 
sub-cesophageal ganglia, but no cerebral ganglia ; 
see Cuvier, Garner, and Rymer Jones, loc. cit. 
For the neryous system of the Pteropoda, see 
also Souleyet, Comp. rend. XVII. No. 14; or 
Froriep’s neue Not. XXVIII. p. 84. f 
6 With Bullaea, Doridium, and Phyliidia, 
there are two cerebral ganglia united by a more or 
less long commissure ; while with Tritonia, and 
Scyllaea, there are four united by short commis- 
sures; see Cuvier, loc. cit. ‘With dAeolis, the 
cerebral mass is also composed of four ganglia 
transversely arranged (Delle Chiaje, Descriz. 
loc. cit. Tav. LXXXVIII. fig. 12,15, and Han- 
cock and Embleton, loc. cit. Pl. V. fig. 16). With 
5 This form is found ‘especially i in those sp 
where the eyes and tentacles are abortive or en- 
tirely wanting ; see Van Beneden, Exercices zoot. 
Fasc. II. (Hyalea, Tiedemannia, Cleodora, Cu- 
vieria, Limacina, and Cymbulia). Probably to 
the absence of these organs is due, with the Ptero- 
poda, the often confounding of the dorsal with the 
ventral surface. It is, moreover, interesting that, 
among the Gasteropoda, Chiton, which is without 
* [ § 208, note 3.) For the cutaneous nerves 
and their mode of distribution with Carinaria, 
sce Leydig (Siebold and Kélliker’s Zeitsch. III. 
1851, p. 325). Here, the nerves branch into finer 
and finer filaments, and finally lose themselves in 
a terminal net-work ; these terminal branches have 
frequent ganglionic corpuscles in their course. 
These corpuscles appear to be developed in the 
nerve-tube ; see loc. cit. Taf. IX. fig. 5. —Ep. 
t (§ 208, note 5.) Middendorff (loc. cit. p. 75) 
has described with Chiton a flat and almost per- 
pendicular nervous band situated on the internal 
sphincter of the mouth, and which he thinks is 
probably two ganglia cerebralia fused together 
(Taf. IX. fig. 6,2); this band sends off numerous 
Zephyrina, Amphorina, Pelta, and 
Thatta? there are two pairs of fused ganglia 
which are connected together by a delicate com- 
missure (Quatrefages, Ann. d. Sc. Nat. XIX. 
1848, p. 293, Pl. XI. fig. 3,4, I. 1844, Pl. VI. fig. 
1-4). With Thetis, and Doris, on the other 
hand, the brain is a single mass, of considerable 
size, and situated in the neck (Cuvier, loc. cit.).t 
Nervi labiales to borders of the mouth (Taf. IX. 
fig. 6, 8). — Ep. 
£ [ § 208, note 6.] Blanchard (Ann. d. Sc. Nat. 
XI. 1849, p. 78) describes the central nervous sys- 
tem of Janus (Eolidia) as consisting of six med- 
ullary masses around the oesophagus, — the cere- 
bral, the cervical and the pedal ganglia; see his 
figures, Pl. III. fig. 1, and Pl. IV. fig. 1. For the 
Cephalic nervous system of the Nudibranchia, see 
Alder and Hancock, |ce. cit. Part II. Pi. IL. fig. 
9 (Dendronotus, Dote\, cerebral ganglia, four, 
and give off ten pairs of nerves; Part III. Pl. 
VIII. fig. 3 (Eolis), cerebral ganglia, four, and 
give off twelve pairs of nerves; Part IV. Pl. V. fig. 
18 (Zumenis), cerebral ganglia, four, and give 
