THE EVIDENCE OF THE ORGANS OF VISION 1 1 7 



Summary. 



The most important discovery of recent years which gives a direct clue to 

 the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland 

 is all that remains of a pair of median eyes which must have been functional in 

 the immediate ancestor of the vei'tebrate, seeing 1 how perfect one of them 

 still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of 

 eyes, one pair situated laterally, the other median. In striking confirmation of 

 the origin of the vertebrate from Palaeostracans it is universally admitted that 

 all the Eurypterids and such-like forms resembled Limulus in the possession of 

 a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient 

 mailed fishes the Ostracodermata, which are the earliest fishes known, are all said 

 to show the presence of a pair of median eyes as well as of a pair of lateral eyes. 

 This evidence 'directly suggests that the structure of both the median and 

 lateral vertebrate eyes ought to be very similar to that of the median and lateral 

 arthropod eyes. Such is, indeed, found to be the case. 



The retina of the simplest form of eye is formed from a group of the superficial 

 epidermal cells, and the rods or rhabdites are formed from the cuticular covering 

 of these cells ; the optic nerve passes from these cells to the deeper-lying brain. 

 This kind of retina may be called a simple retina, and characterizes the eyes, 

 both median and lateral, of the scorpion group. 



In other cases a portion of the optic ganglion remains at the surface, when 

 the brain sinks inwards, in close contiguity to the epidermal sense-cells which 

 form the retina ; a tract of fibres connects this optic ganglion with the under- 

 lying brain, and is known as the optic nerve. Such a retina may be called 

 a compound retina and characterizes the lateral eyes of both crustaceans and 

 vertebrates. Also, owing to the method of formation of the retina by invagina- 

 tion, the cuticular surface of the retinal sense-cells, from which the rods are 

 formed, may be directed towards the source of light or away from it. In the 

 first case the retina may be called upright, in the second inverted. 



Such inverted retinas are found in the vertebrate lateral eyes and in the 

 lateral eyes of the arachnids, but not of the crustaceans. 



The evidence shows that all the invertebrate median eyes possess a simple 

 upright retina, and in structure are remarkably like the right median or pineal 

 eye of Ammocoetes ; while the lateral eyes possess, as in the crustaceans, an 

 upright compound retina, or, as in many of the arachnids, a simple inverted 

 retina. The lateral eyes of the vertebrates alone possess a compound inverted 

 retina. 



This retina, however, is extraordinarily similar in its structure to the 

 compound crustacean retina, and these similarities are more accentuated in the 

 retina of the lateral eye of Petromyzon than that of the higher vertebrates. 



The evidence afforded by the lateral eye of the vertebrate points unmistakably 

 to the conclusion that the ancestor of the vertebrate possessed both crustacean 

 and arachnid characters — belonged, therefore, to a group of animals which gave 

 rise to both the crustacean and arachnid groups. This is precisely the position 

 of the Palfeostracan group, which is regarded as the ancestor of both the 

 crustaceans and arachnids. 



