150 THE SENSES OF ANIMALS 



From this simple type of eye a host of progressively more 

 complicated types can be found in other species. In a slightly 

 more advanced type the pit is a partly closed cup filled 

 with jelly which is in contact with the water at the opening 

 through which light enters; in the next stage the opening 

 is closed by transparent skin ; in another, such as the snail's, 

 the jelly is much firmer and forms a lens separated from 

 contact with the retina. Judging from the structure of the 

 more complex types of eyes we should expect the animals 

 possessing them to have some considerable power of sight. 

 Experiment, however, has failed to show such vision and it 

 is very doubtful if the eyes function as more than light- 

 gatherers; if there is any vision the species that enjoy it 

 must be extremely short sighted. On the other hand, the 

 whole surface of the body of some of these creatures is sensi- 

 tive to light, and animals deprived of their eyes react to a 

 shadow falling upon them. 



In the cephalopods — the octopuses, squids and cuttles — 

 there is a similar increase in complexity of the eye from a 

 very simple type. In the nautilus, one of the very few of 

 these animals that has an outside shell, the eye, like that of 

 the limpet, is an open cup lined with a retina but without 

 a lens. In the most complex types, such as the octopus and 

 squid, the eye is remarkably like that of a mammal, and 

 has lens and iris, focusing arrangements, anterior and 

 posterior chambers, and a well-developed retina in which 

 the nerve fibres are not spread over the surface of the light 

 receptors but run below them. These eyes are very efficient 

 image-formers and the behaviour of the animals shows that 

 they have very good eyesight indeed. Their brains, how- 

 ever, are very different in structure and comparative size 

 from those of mammals, and it is probable that the picture 

 of the surroundings built up in the brain is by no means 

 similar to that which we should build up from the same 

 images on our retinas. 



In the other molluscs — those with two shells, such as 

 cockles and mussels — the eyes are very different. In the 

 first place these animals have no head and their range of 

 locomotion is comparatively restricted; indeed some are 

 permanently fixed to their surroundings and cannot shift. 



