THE MOLLUSC A 



19. 



Gastropod, Ontidium, 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 Nautilus (Fig. 6, A), or with a crystalline lens, as in Pleuroto- 

 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 



G.op 



N.op 



FIG. 6. 



Diagrams of sections of the eyes of Mollusca. A, Nautilus ; B, Gastropod (Limax or Helix) \. 

 C, Dibranchiate Cephalopod (Oigopsid). Co, external cornea ; Co.ep, internal cornea ; G.op, 

 optic ganglion ; Int, Int 1 , Int-, Int 3 , different parts of the integuments ; Ir, iris ; I, crystalline 

 lens ; I 1 , 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 Grenadier.) 



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 (Euthyneura), in some genera of Streptoneura, 

 in one order (Anatinacea), and in some isolated species of Lamelli- 



