i 3 2 THE LIFE OF THE MOLLUSCA 



Mollusca is limited. The Cephalopods and Gastro- 

 pods afford instances of progressive development in 

 this organ. In Nautilus 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 Naticidae, Bul- 

 lidse, the Pleurobranchidse, and some Pulmonates) or 

 to be wholly wanting (sundry species of NaticA, 

 Terebra, of the Olividse, Bullia, some subterranean 

 Pulmonates, certain abyssal Gastropods, internal 

 parasites like the Entoconchidse, and pelagic forms 

 such as Ianthina and the Pteropods). 



Among the Bivalves, except the Mytilidse and 

 Pteria, true cephalic eyes are wanting in the adult, 

 though present in some larvae. 



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 



