234 



THE CEPnALOPIIORA. 



^208. 



The nervous centre of the genus Sagitta is in many respects like that 

 of the Heteropoda. A hexagonal cerebral gcfnglion lies upon the upper 

 surflice of the oesophagus ; another ((uitc larue is situated in the centre of 

 the ventral surface of the trunk. These intcrconununicate by two large, 

 very long cords. The cerebral ganglion gives oif two pairs of nerves; 

 — of these the anterior arc 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 oft' 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 oesophagus. *^^ 



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 oesophageal ring consists of a simple nerv- 

 ous cord. The two or four cerebral ganglia are either connected by trans- 

 ■"erse commissures, or intimately blended together. '''' 



p. 99, Tav. LXIII. (Carinaria). Fterotrachea 

 has a sirailai- disposition. Accordiiifj to Delle 

 Ckiaje (loc. cit. Tav. 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 oesophagus of Carinaria ami Fterotra- 

 chea ; but this is not mentioned by either Cuvier, 

 or Milne Edamrds. 



3 See Krohn, loc. cit. p. 12, fig. 2, 5, 13.* 

 i With Apli/fsia, according to Cuvier, Mem. loc. 

 cit. p. 22, PI. III. IV., and with Pleurobranchus, 

 according to Delle Cliiaje, Memor. loc. cit. Tav. 

 XLI. fig. 8, o. v. v. I have found the oesophageal 

 ring arranged in the same way with Pleuro- 

 branchaea. 



5 This form is found especially in those species 

 where Ihe eyes and tentacles are abortive or en- 

 tirely wanting ; see Van Beneden, E.Kercices zoot. 

 Fasc. II. (liyalca, Tiedemannia, Cleodora, Cu- 

 vieria, Limacina, and Cijmbulia). 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.J For the cutaneous nerves 

 and their mode of distribution with Carinaria, 

 see Leydic; (Siebold and Kiilli/cer'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. — Ed. 



t [ § 208, note 5.J Middendorf (loc. cit. p. V5) 

 has described with Chiton a flat and almost per- 

 I(endicular nervous band situated on the internal 

 sphincter of the mouth, and which he thinks is 

 probably two ganglia ccrebrnlia fused together 

 (i'.if. IX. fig. 6, a) ; this band sends ofif numerous 



eyes and tentacles, has a transversal row of six 

 sub-ceso])hageal ganglia, Ijut no cerebral ganglia ; 

 see Cuvier, Garner, and Rymer Jones, loc. cit. 



For the nervous system of the Pteropoda, see 

 also Souleyet, Comp. rend. XA'II. No. 14 ; or 

 Frnriep's neue Not. X.XVIII. p. 84. t 



H With Bullaea, Doridium, and Phyllidia, 

 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 Aeolis, the 

 cerebral mass is also composed of four ganglia 

 transversely arranged {Delle Chinje, Descriz. 

 loc. cit. Tav. LX.XXVIII. fig. 12, 15, and Han- 

 cock and Embleton, loc. cit. PI. V. fig. 10). With 

 Eolidina, Zephyrina, Amphorina, Pelta, and 

 Chalidis, there are two pairs of fused ganglia 

 which are connected together by a delicate com- 

 missure {Quatrefages, Ann. d. So. Nat. XIX. 

 1843, p. 2c)3, PI. XI. fig. 3, 4, I. 1844, PI. 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.).J: 



Nervi labiates to borders of the mouth (Taf. IX. 

 fig. 6, fi). — Ed. 



X [ § 208, note 6.] Blanchard (Ann. d. Sc. Nat. 

 XI. 184',), p. 78) describes the central nervous sys- 

 tem of Janus {Eolidia) as consisting of six med- 

 ullary masses around the a?sophagu's, — the cere- 

 bral, the cervical and the pedal ganglia ; see liis 

 figures, PI. III. fig. 1, and PI. IV. fig. 1. For the 

 Cephalic nervous system of the Nudibranchia, see 

 Alder and Hancock., loc. cit. Part II. PI. II. fig. 

 9 (JJendronotus, Doto), cei-ebral ganglia, four, 

 and give off ten pairs of nerves ; Part III. PI. 

 VIII. fig. 3 (Eolis), cerebral ganglia, four, and 

 give off twelve pairs of nerves ; Part IV. PI. V. fig. 

 13 {Euvienis), cerebral ganglia, four, and give 



