240 THE CEPHALOPHORA. § 212. 
There is, moreover, a series of Cephalophora with which the eyes are 
much more 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, Rostellaria, 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. According to Muller (Ann. d. Sc. Nat. 
loc. cit. p. 12, Pl. III. fig. 5), the optic nerve is 
only a special branch of the end of the tentacular 
nerve, 
8 The two pretty simple eyes of Sagitta, forming 
two prominences on the top of the head, are spher- 
ica}, and rest directly upon the ganglionic enlarge- 
ment of the optic nerve , see Krohn, loc. cit. p. 13, 
fig. 5, 14. 
9 According to Quatrefages (loc. cit. I. p. 158, 
PL VI. fig. 6, 7), the eyes of Pelta, and :Chalidis, 
have, instead of a choroidea, a mass of pigment 
containing neither a sclerotica nor a cornea. Ac- 
cording to the observations of Nordmann, and 
Kélliker, the eyes of Tergipes and Polycera are 
without optic nerves, and lie directly in contact 
with the cerebral ganglia. With Doris, Glaucus, 
Thetis, Aeolis, Doridium, 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, Eumenis, 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. — Ep. 
t [ § 212, note 13.] See, in this connection, Lespes 
(Recherches sur l’oeil des Mollusques Gastéropodes 
terrestres et fluviatiles de France, Thesis. Tou- 
louse, 1851). His conclusions are : 
“1. All the terrestrial and fluviatile Gastero- 
poda have eyes 5 
“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, Pectinibranchia, and with some of 
the Heterobranchia; 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, Emarginula, Fissurella, Siga- 
retus, Paludina, Littorina. A kind of peduncle~ 
replaces this prominence with Haliotis, Navicella,- 
Phasianella, Trochus, Ceratodes, Ampullaria. 
12 The 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 Strombus. 
13 Amphipneusta, Helicina, and Limacina.t 
“(1.) The eye at the extremity of the tentacle 
(Heliz) ; 
“(2.) The eye at the internal base of the tenta- 
cle (Limnaea) ; 
“(3.) The eye at the external base of the tentae 
ele (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.” — Ep. 
