50 Shell Life 



From a consideration of a series of molluscan eyes 

 it is not difficult to surmise the developmental history 

 of the most perfect among them. Thus, in the 

 Limpet — to which we have already referred as being 

 probably much like the ancestral mollusk — there is 

 little more than a fold of the skin forming a depres- 

 sion lined with rod-shaped cells containing pigment 

 and forming a retina, which is connected with the 

 optic nerve. In the Tops {Trochus) the depression 

 of the epidermis is greater, so that it encloses a 

 spherical space lined with retinal cells and filled 

 with fluid (vitreous humour), but open in front so 

 that the face of the eye is bathed in water. In the 

 Sting Winkles (Murex) there is an advance upon this 

 form, for the opening of the eye has become quite 

 closed and there is a lens before the vitreous humour. 

 Now the eyes of these three species present a very 

 close resemblance to three stages in the development 

 of the eye in the embryo of the Squid (Loligo), which 

 shows an advance upon Murex by producing an iris 

 round the fore-part of the lens, an optical chamber in 

 front of the lens, and a transparent cornea before that 

 again. 



It must be apparent that eyes are of the greatest 

 value to the most active species of Mollusca — the 

 Cuttles. These have, consequently, developed their 

 eyes to a greater degree of efficiency than other 

 orders. The inactive, often fixed, bivalves have been 

 found in many cases to have a pair of eyes on what 

 may be regarded as the head during their larval 

 condition, thus indicating that in the early history 

 of the Pelecypoda they had eyes in the adult state. 

 Those bivalves that lead a more active life, such as 



