THE MOLLUSCA 19 
Gastropod, Oncidiwin, possesses, in addition to a pair of normal 
cephalic eyes, numerous pallial eyes scattered over the whole dorsal 
surface. 
The cephalic eyes are pigmented invaginations of the integu- 
ment: they may be open, without any refractive body, as in Patella 
and Nuutilus (Fig. 6, A), or with a crystalline lens, as in Plewroto- 
maria, Trochus, Haliotis, etc. They are closed, and have a cornea 
and an internal crystalline lens in the majority of Gastropods and 
in the dibranchiate Cephalopods (Fig. 6, B, C). The pallial eyes may 
be “compound,” without an internal crystalline lens (Arcidae), or 
simple. In the latter case they may present one of the following 
characters :—(1) an internal crystalline lens and a deep retina 
(Polyplacophora, Fig. 33); (2) an internal crystalline lens and a 
we 
Gop 
Fic. 6. 
Diagrams of sections of the eyes of Mollusca, A, Nautilus; B, Gastropod (Limaa or Helix) ; 
C, Dibranchiate Cephalopod (Oigopsid). Co, external cornea; Co.ep, internal cornea; G.op, 
optic ganglion ; Int, Intl, Int, Int3, different parts of the integuments ; Ir, iris ; /, crystalline 
lens ; ie outer (extra- corneal) ‘portion of the lens ; N.op, optic nerve ; N.S, nervous stratum of 
the retina ; Pal, eyelid ; x, inner layer of the retina. (From Lankester, after Grenacher.) 
superficial retina (Pecten, Fig. 217); (3) an internal crystalline 
lens, the retinal cells reversed and the nerve traversing the retina 
(Oncidium). With the exception of the Cephalopods, and 
possibly also the Heteropods, the vision of Molluscs is limited. 
In the forms devoid of eyes, as in other groups of the animal 
kingdom, the general surface of the body is capable of dermatoptic 
perceptions. 
7. Organs of Generation.—Among Molluscs in general the sexes 
are separate, and this is the case in the most archaic forms of the 
different groups of the phylum. Hermaphroditism, on the contrary, 
is always a sign of specialisation, and is only found as a normal 
condition in one sub-order of Amphineura (Neomeniomorpha), in one 
sub-class of Gastropoda (Kuthyneura), in some genera of Streptoneura, 
in one order (Anatinacea), and in some isolated species of Lamelli- 
