^212. 



THE CEPHALOPHORA. 



239 



§ 212. 



The organs of Vision are absent with only a very few genera of the Ce- 

 phalophora.'^^ They are never more than two in number, and their size, com- 

 pared to that of the bod}', is usually small ; they are smallest with some 

 Heterobranchia, and the largest with the Pectinibranchia.*^* 



The eyes consist usually of two round bulbs concealed under the skin ; 

 this last is colorless at this point, and lies over them like a thin lamella. Each 

 bulb is limited outwardly by a tissue resembling a Sclerotica, hut beneath the 

 skin, this tissue is more convex than elsewhere, and thus forms a kind of Cor- 

 neaf'^ The sclerotica is lined by a dark pigment layer, or Choroidea, which, 

 near the corner, ends in a free border, forming thus a Pupilla. With some 

 Gasteropoda, the pupillary border has a very thick pigment layer which 

 serves, perhaps, as an Iris.^*' 



The internal surface of the choroidea is covered by a whitish pellicle 

 which undoubtedly is a Retina, for the optic nerve enters the sclerotica at 

 a point opposite the cornea. ^'^ The cavity of the eye-bulb is filled with a 

 gelatinous, vitreous body, which, in front, envelops a spherical crystalline 

 lens.(«> 



The Optic nerve arises from the cerebral ganglia, and runs along, for a 

 longer or shorter distance, in company with the tentacular nerve of the 

 same side.^*^' 



ere. zoot. Fasc. II. p. 13, PI. I. fig. 8, f. 9, c. 10, 

 PI. V. fig. 13, X.) affirms that with the first of these 

 genera, and with Tiedemannia, and Limacina, the 

 auditive vesicles lie directly upon the two princii)al 

 inferior ganglia ; this agrees with Delle Chiaje's 

 description of these organs with Cyrnbu/ia ; see his 

 Descriz. &c. I. p. 94, Tav. XXXII. fig. 2, i. 

 Eschrickt (loc. cit. p. 6, Tab. III. fig. 28, s.) has 

 figured, with a Clio, two gangUa with short pe- 

 duncles, situated close by the two anterior ganglia 

 of the oesophageal ring. These, I infer, are only 

 the auditive capsules receiving two short auditive 

 nerves. ' 



With those Gasteropoda whose inferior oesophageal 

 ganglia are arranged in a circle, as, for examples, 

 with Lymnaeus, Planorbis, Physa, Succinea, 

 Bulimus, Ancylus, these capsules lie upon the 

 posterior surface of the two large anterior ganglia. 

 But when, on the other hand, these ganglia are 

 approximated, or even fused into one common mass, 

 as with Helix, these capsules lie upon the inferior 

 surface of this mass, and especially ujjon the 

 nodules corresponding to two large anterior ganglia. 



1 Pfiyllirrhoe, Diphyllidia, Chiton, Deniali- 

 um, and the Pteropoda with the exception of Sa- 

 gitta and Clio, are bUnd. In many of the Ptero- 

 poda, the auditive appear to have been taken for 

 the ocular organs. 



Si Swammerdamm (Bibel der Nat. p. 47, Tab. IV. 

 fig. 5-8 ■) ma^le out very correctly the structure of the 

 eyes of Helix. The later works of Stie.bel {Meck- 

 el's Deutsch. Arch. 1819, p. 296, Tab. V.), Huxchke 

 (Beitr. zur Physiol, u. Natmgesch. 1824, p. 57, 

 Taf. III. fig. 8), and oi De Blainville (De TOrgan- 

 isat. des Animaux, 1823, p. 445), upon the eyes of 

 Helix, Paludina, and Valuta, liave been much 

 improved by those of Mailer {MeckePs Arch. 



1829, p. 208, Taf. VI. fig. 4-8, and Ann. d. So. 

 Nat. XXII. 1831, p. 7, PI. III. IV., or in the Isis, 

 1835, p. 347, Taf. VII.), and Krohn {Muller\t 

 Arch. 1837, p. 479, 1839, p. 332, Taf. X. fig. 6-8) 

 upon the eyes of Helix, Murex, Paludina, and 

 Pterotrachea. 



3 The eyes of the Ileteropoda present a remark- 

 able exception ; their very convex cornea is svu'- 

 rounded by a collar of skin ; the ocular bulb is very 

 long, and, at its base, the sclerotica spreads out 

 interiorly and posteriorly, forming a round prom- 

 inence ; see Milne Edwards, Ann. d. Sc. Nat. 

 XVIII. 1842, PI. XI. fig. 1, c. (Carinaria), and 

 especially the description of Krohn (loc. cit. 1839) 

 of the eye of Pterotrachea. The ocular bulb of 

 Clio is also very long, but has no prominence ; see 

 Eschricht, loc. cit. p. 7, Tab. III. fig. 29. Those 

 of Artaeon are long and jiyriform ; see Quatre- 

 fages, Ann. d. Sc. Nat. I. 1844, PI. VI. fig. 5, and 

 Allman, loc. cit. PI. VII. fig. 2. 



•t A dark iris is distinctly seen with Paludina 

 and Murex. That of Strombus is very brilliant 

 and multicolored, according to Quay and Gai- 

 mard ; see, Voy. de l'.\strolabe, Zool. 111. p. 56, Mol- 

 lusques, PI. L. LI. I atn not yet certain whether 

 or not the iris of these Gasteropoda is susceptible 

 of movements of contraction and dilatation. It 

 may be well to add that the choroidea of the Ile- 

 teropoda lias several spots of its surface free from 

 pigment.* 



a Krohn (loc. cit. 1837, p. 482) affirms that he 

 has seen this white layer with a Priludina. 



<l The existence of a distinct vitreous body was 

 known to Swammerdamm,a,UiX lias been confirmed 

 by Krohn (loc. cit. 1837). 



1 According to Krohn (loc. cit. 1839), the two 

 optic nerves of Paludina, Murix, Aplysia, Cy- 



* [ § 212, note 4.J For the visual organs of Pa- 

 ludina, with histological details, and especially 

 confirmatory of Krohn's observations, see Leydig, 

 loc. cit., Siebold and Kiilliker's Zeitsch. II. 1850, 



p. 159, Taf. XII. fig. 25, Taf. XIII. fig. 26-28. 

 See also this same author in Siebold and Kolli- 

 ker''s Zeitsch. 1851, III. p. 327 (Cartnarjo). — Ko. 



