THE SENSE ORGANS 



581 



nerve fibers with the brain. In some, the beaker-like eyes are connected 

 with the epidermis; in others the eye sinks below the skin. Nereis has 

 vesicular eyes. The differences between the various types of annelid eyes 

 are so great that it is impossible to believe that they are genetically related 

 to one another. 



The eyes of molluscs are sometimes beaker-eyes, sometimes vesicular 

 eyes with a lens. On the edge of the mantle of pecten, are complex vesic- 

 ular eyes, which have many features in common with those of vertebrates. 

 Each eye is partly surrounded by a layer of opaque pigmented epithelium, 

 which in front of the eye becomes a translucent cornea. Beneath the 



CARTILASE 



HUSCUE 



Fig. 479. — A diagram of median sections of the eyes of vertebrates and cuttlefish. 

 While the two types of eye resemble one another in many fundamental characters, the 

 retina of the vertebrate eye is inverted, while that of the cuttlefish is not. Bergson 

 has challenged the assumption of Darwinians that two eyes of such divergent origin 

 could by chance variations alone come to resemble one another so closely. 



cornea and adherent to it, is a biconvex lens. Like the vertebrate eye, 

 that of Pecten has a liquid-filled chamber. But the retina of Pecten, 

 unlike that of vertebrates, has two layers of photoreceptors, each photo- 

 receptor connected with a nerve fiber. Back of the retina lies an inner 

 layer of pigment epithelium. There is no reason to doubt that such an eye 

 is able to form an image. 



The eyes of cephalopods, especially those of sepia and loligo, bear a 

 striking resemblance to those of vertebrates. But the fact that the retina 

 is not inverted as in vertebrates, proves that the two cannot be genetically 

 related. Bergson has cited this instance of similar results, notwith- 

 standing diverse conditions, as proof of the existence of a vital factor or 

 "elan vital" which distinguishes the living from the lifeless. Since Berg- 

 son appears to be substituting one mystery for another, the elan vital 

 hypothesis has not made a very strong appeal to biologists. Nevertheless, 

 it is equally difficult to accept the assumption that eyes so alike could 

 have been independently produced merely by the selection of random 

 variations of so many elements as enter into the composition of cephalopod 

 and vertebrate eyes. 



