242 ANIMAL PEDIGREES 



spherical transparent lens of the eye ; and the 

 development is completed by histological changes 

 in the inner wall of the vesicle, which convert it 

 into the retina, and by the formation of folds of 

 skin around the eye, which become the iris and the 

 eyelids respectively. Each stage of this develop- 

 mental history is a distinct advance, physiologically, 

 on the preceding stage, and furthermore, each 

 stage is retained at the present day as the perma- 

 nent condition of the eye in some member of the 

 group Mollusca. The earliest stage, in which the 

 eye is merely a slightly depressed and slightly 

 modified patch of skin, represents the simplest 

 condition of the Molluscan eye, and is retained 

 throughout life in Solen. The stage in which the 

 eye is a pit with widely open mouth, is retained 

 in the limpet ; it is a distinct advance on the former, 

 as through the greater depression the sensory cells 

 are less exposed to accidental injury. The narrow- 

 ing of the mouth of the pit in the next stage is a 

 simple change, but a very important step forward. 

 Up to this point the eye has served to distinguish 

 light from darkness, but the formation of an image 

 has been impossible. Now, owing to the smallness 

 of the aperture, and the pigmentation of the walls 

 of the pit which accompanies the change, light from 

 any one part of an object can only fall on one 

 particular part of the inner wall of the pit or 

 retina, and so an image, though a dim one, is 

 formed. This type of eye is permanently retained 

 in the Nautilus. The closing of the mouth of the 

 pit by a transparent membrane will not affect the 



