240 



THE CEPnALOPHORA. 



<^212. 



There is, moreover, a series of Cephalophora with which the eyes are 

 much luoie simple and often nearly abortive. Such is the case with Sagitta, 

 and many of the Apneusta and Heterobranchia.'*^* Here, the eyes are 

 not always nicely limited by a sclerotica, but the light-refracting bodies lie 

 surrounded in a mass of pigment granules, and situated more or less dis- 

 tant from the external surface of the cervical region. The cornea is absent, 

 and often also the optic nerve, in which case, the eyes lie directly upon the 

 cerebral mass.^''^^ 



The most complete eyes are nearly always connected with the tentacles, 

 although their position varies quite widely. '^"^ Very often they are situated 

 at the base of the external surface of these organs. ^^" "With many Pectini- 

 branchia, they are more or less elevated upon the outer side of the ten- 

 tacle on a protuberance or on a support which exceeds the extremity of the 

 tentacle in length and size.^'-' With many Pulmonata, these organs are 

 situated upon the very extremity of the tentacles, and are upon the pos- 

 terior pair, when these last are four in number. ^^'^^ 



praea, Rostellnria, Buccinum, and Littorina, 

 arise from the cerebral ganglia by an origin which 

 is distinct from that of the tentacular nerves. I 

 have been able to confirm this for Helix, Limax, 

 Caracolla. Accoi-ding to Miiller (Ann. d. Sc. Nat. 

 loc. cit. p. 12, PI. III. fig. 5), the optic nerve is 

 only a special branch of the end of the tentacular 

 nerve. 



'i The two pretty simple eyes of Sasritta, forming 

 two prominences on the top of the head, are spher- 

 ical, and rest directly upon the ganglionic enlarge- 

 ment of the optic nerve , see Krnhn, loc. cit. p. 13, 

 fig. 5, 14. 



i) According to Quatrefages (loc. cit. I. p. 158, 

 PI. VI. fig. 6, 7), the eyes of Pelta, and C/ialidis, 

 have, instead of a choroidea, a mass of pigment 

 containing neither a sclerotica nor a cornea. Ac- 

 cording to the observations of JSordmann, and 

 KiiUiker, the eyes of Tergipes and Polycera are 

 without optic nerves, and lie directly in contact 

 with the cerebral ganglia. With Doris, Glaucus, 

 Thetis, Aeolis, JJoridium, Aplysia, Bulla, Bul- 



* [ § 212, note 9.] For the eyes and their inti- 

 mate structure with the Nudibranchia, see Alder 

 and Hancock, loc. cit. (Dendronotus, Doto, Aeo- 

 lis, Scyllaea, Eumcnis, Doris, Antiopa) ; with all 

 these, the optic nerves were distinct, and the eye 

 itself was furnished with a well-rounded, black pig- 

 ment-cup, often a spherical crystalline lens (Doris, 

 Aeolis, Antiopa), with an arched cornea in front, 

 and the whole enveloped by a transparent mem- 

 branous capsule. — Ed. 



t [ § 212, note 13.] See, in this connection, Lespis 

 (Recherches surl'oeildes MoUusques Gast^ropodes 

 terrestres et fiuviatiles de I'rance, Thesis. Tou- 

 louse, 1851). His conclusions are : 



" 1. All the terrestrial and fluviatile Gastero- 

 poda have eyes ; 



" 2. These organs present, as to their position, 

 three different types : 



laea, &c., these organs are comparatively small 

 and appear more or less distinct through the skin, 

 sometimes in front of, and sometimes behind, the 

 tentacles.* 



10 This is so with various Heteropoda, all the 

 Pulmonata, Pectinihranchia, and with some of 

 the Ileterobranchia ; see Loven, loc. cit. and 

 Isis, 1842, p. 364. 



11 The eyes are situated at the base of the tenta- 

 cles on a small prominence, with Carinaria, At- 

 lanta, Vermetus, with the Lymnaeacea, the Oper- 

 culata. Patella, Emarginitla, Fissurella, Siga- 

 retus, Paludina, Littorina. A kind of peduncle 

 replaces this prominence with Haliotis, Navicella, 

 Phasianella, Trochus, Ceratodes, Ampullaria. 



12 Tlie prominences supporting the eyes are situ- 

 ated on the outer side of the tentacles with Bucci- 

 num, Harpa, Dolium, Cypraea, Murex, Oliva, 

 Turbo ; and at a variable distance from the ex- 

 tremity which they sometimes surpass in breadth 

 and length, as, for example, with S trombus. 



13 Amphipneusta, IleUcina, and Limacina.f 



"(1.) The eye at the extremity of the tentacle 

 (Helix) ; 



" (2.) Tlie eye at the internal base of the tenta- 

 cle (Lirnnaea) ; 



" (3.) The eye at the external base of the tenta- 

 cle (Cyclostoma). 



"3. These organs present also three types as to 

 their organization : 



"(1.) The lenticular crystalline lens, the vitre- 

 ous humor fluid, non-adherent (Helix) ; 



"(2.) The lenticular crystalline lens, the vitreous 

 humor thick and united to this last ; 



" (3.) The crystalline lens thick and slightly con- 

 vex, the vitreous humor viscous and slightly ad- 

 herent to the lens." — Ed. 



