OKGANS OF VISION. 505 



It cannot be asserted even that all paired cephalic eyes are 

 homologous, although it is possible that they originate from a 

 common but very simple ancestral type, since such a type is 

 unknown. The simplest eye spots, amongst the Annelida, 

 exhibit a very different structure, and are often more probably 

 the ancestral form of median cephalic eyes, than of the more 

 highly developed paired cephalic eyes ; the simple eye-like 

 structures of the Ascidian, and of Amphioxus, and some 

 Crustacea are more like the median than the paired cephalic 

 eyes, which in the two former cases at least they undoubtedly 

 represent. 



In Insects all three forms of eye are, I believe, represented, 

 for if the remarkable eye-like organs of the Blow-fly larva, 

 which are situated on the maxillae, are eyes at all, they are un- 

 doubtedly sporadic, and it is said that in the larva of Miastor 

 there is a sporadic eye on the fourth segment of the body. 

 The median ocelli, or simple eyes, of many imagines are median 

 cephalic eyes, and the great compound eyes are paired cephalic 

 eyes. 



The Researches of Patten on the eyes of Area, amongst the 

 Lamellibranchiata, do much to break down the distinction 

 between simple and compound eyes ; and even in Arthropods 

 it does not appear unlikely that both have been derived from a 

 common ancestral form. 



Kolliker [242] has classified the visual organs of animals 

 under three groups : 



(1) True brain eyes (echte Hirnaugen), which he defines as 

 ' eyes developed entirely from the medullary plate' the eyes of 

 the Ascidian larva and the pineal eyes of Vertebrates. 



(2) Epiblastic eyes, which consist entirely of ectodermal 

 elements, derived from the superficial epiblast the eyes of 

 Invertebrates generally. 



(3) Eyes developed in part from the medullary plate, and in 

 part from the superficial ectoderm the paired eyes of Verte- 

 brates. 



The Pineal Eye. With regard to Kolliker's first class, ' echte 

 Hirnaugen.' It is indubitable that the nervous elements of the 



