HOW CRABS SEE 61 



HOW CRABS SEE 



Perhaps, too, you would like to know some- 

 thing about the eyes of crabs ; for these crea- 

 tures see in a very odd way. On each side of 

 the head is a kind of stalk, something like those 

 which you may see on the heads of slugs and 

 snails, only very much smaller. And at the tip 

 of each stalk is a small black spot. Now if 

 you were to put one of these little stalks under 

 the microscope, and to look at the black spot, 

 you would find that it was made up of hundreds 

 and hundreds of very tiny eyes, very much like 

 those of insects, except that instead of being 

 six-sided they are square. So that altogether, 

 perhaps, a crab may have three or four thousand 

 eyes, or even more ! 



That sounds a very large number, doesn't it? 

 But then, you see, a crab cannot move its eyes 

 up and down, and from side to side, as we can. 

 They are fixed, and cannot be moved at all. 

 Each eye, however, looks in rather a different 

 direction from all the rest. Some eyes look 

 upwards, some look downwards, some look for- 

 wards, some look backwards, and some look 

 out on either side. So without moving its head 

 at all the crab is able to see all round it. 



