196 PROFESSOR E. EAY LANKESTER AND A. G. BOURNE. 



Subsequently, as it would seem, to the segregation of the 

 layer of nerve-end cells into retinulse, optical advantage may be 

 found in the segregation of the cuticular lens often called cornea. 

 Thus, from the so-called unicorneal the multicorneal condition 

 of eye is developed. It seems to be undesirable to speak of the 

 cuticular lens as a " cornea," with which it has little analogy, 

 if by cornea we understand in the first instance the vertebrate 

 cornea. It will be therefore best to distinguish the simple 

 one-lensed eyes as " monomeniscous," that with a segregated 

 lens as "polymeniscous." This third alternative of structure 

 is presented by Arthropod eyes, which differ in other respects 

 inter se, but it seems that a non-retinulate eye cannot be 

 j polynieniscous, since the segregation of retinulse is the deve- 

 ^ I lopmental antecedent of the segregation of the lens. Hence 

 we may have and actually can point to monostichous polyme- 

 niscous eyes (lateral eyes of Limulus) as well as diplostichous 

 polymeniscous eyes, but all non-retinulate eyes are monome- 

 niscous. 



In this way we are led to correct the conception of the so- 

 called compound or polymeniscous eye which is current, and 

 and is thus enunciated by Gegenbaur (' Comp. Anatomy,' 

 English translation, p. 266) : " A reduction of the retinal ele- 

 ments of the simple eye gives rise to the retinula, and a com- 

 pound eye is formed by the gradual concrescence of a number 

 of simple eyes." On the contrary, it seems that the compound 

 eye is formed, not by the gradual concrescence of a number of 

 simple eyes, but by the segregation of the elements of a simple 

 eye, which affects first the retina and then the lens. It 

 appears from a consideration of the structure of the polymenis- 

 cous lateral eyes of Limulus that even the groups of mono- 

 meniscous lateral eyes found in Scorpions (and similarly, also, 

 the closely-set groups of monomeniscous eyes of Myriapods, 

 which can in some cases be regarded as one large polymeniscous 

 eye) must be looked upon as resulting from a process of segre- 

 gation carried further than is necessary for the production of a 

 compound eye — carried so far, in fact, as to produce from one 

 original large simple (monomeniscous) eye, not a continuous 



