132 THE LIFE OF THE MOLLUSCA 
Mollusca is limited. The Cephalopods and Gastro- 
pods afford instances of progressive development in 
this organ. In Nawtilus and in the Limpet the eyes 
are simple pigmented infoldings of the integument 
without any refractive body (Plate XXIX., Fig. 12, C). 
A crystalline lens is present in the more archaic 
rhipidoglossate Gastropods, and in the rest of the 
Gastropoda this is enclosed by a cornea (Plate 
XXIX., Fig. 12, B). A cornea—incomplete in the 
Oigopsida (Plate XXIX., Fig. 12, A), but complete 
in the Myopsida—is also present in the dibranchiate 
Cephalopoda, while the Octopus, in addition, pos-_ 
sesses an eyelid. In those cases in which the eye 
becomes buried in the integuments, or the animal 
is abyssal or subterranean in habitat, this organ 
tends to become rudimentary (certain Naticidz, Bul- 
lide, the Pleurobranchidz, and some Pulmonates) or 
to be wholly wanting (sundry species of Natica, 
Terebra, of the Olivide, Bullia, some subterranean 
Pulmonates, certain abyssal Gastropods, internal 
parasites like the Entoconchidz, and pelagic forms 
such as Ianthina and the Pteropods). 
Among the Bivalves, except the Mytilide and 
Pteria, true cephalic eyes are wanting in the adult, 
though present in some larve. 
Accessory eyes are developed in some molluscs in 
other parts of the body. In certain species of Chiton 
scattered over the shell are many thousands of eyes, 
some with and some without crystalline lenses ; they 
