394 COMPARATIVE ZOOLOGY OF CHORDATES 



of the brain. It is the fibres from the median portions of the 

 retinae which cross-over and go to the opposite side of the 

 brain. The images of one object fall on corresponding points 

 in the two retinae, and the fibres from these corresponding 

 points run to one and the same side of the brain. 



Many of the lower vertebrates have been shown to be 

 sensitive to different colours. It is supposed that the cones 

 of the retina are sensitive to colour, and that the rods only 

 perceive light and dark. Nocturnal animals such as owls and 

 bats have scarcely any cones, and are presumably colour-blind. 

 The proportion of cones in the retina increases as a rule from 

 the lower to the higher vertebrates. In the higher Primates 

 and in man, the eyes have " corresponding points " of optimum 

 sensitiveness (the macula lutea or " yellow spot "), in which 

 the retina consists of cones only, without any rods. 



In some vertebrates the eyes have been lost. They are 

 very degenerate in some of the Cyclostomes, which lead a 

 semi-parasitic life, and in the Urodele Proteus, which inhabits 

 the dark caves of Carniola. Fish which live in the dark of 

 the abyss of the ocean or in caves may be blind and eyeless, 

 as, for example, Ipnops, Amblyopsis, and Lucifuga. Among 

 mammals, the eyes are often reduced in forms which live in 

 the dark in burrows underground. The common mole is an 

 example, and a comparable but even more far-reaching reduc- 

 tion of the eyes has taken place independently in the " marsupial 

 mole " Notoryctes. 



The Pineal. — There is no doubt that the early vertebrates 

 were capable of seeing by means of their pineal organs, through 

 the pineal foramen in the roof of the skull, though possibly 

 not of forming an image. Among living forms, Petromyzon 

 has two pineal organs ; other forms have only one, which may 

 represent the original right or left organ. The pineal is least 

 degenerate in Sphenodon. It is in the form of a vesicle of 

 which the upper wall forms the lens and the lower the retina, 

 which is connected by nerve-fibres with the brain. This 

 retina is not " inverted," as is that of the paired eyes. Sur- 

 rounding the retina is pigment, and the organ is sensitive 

 to light. 



