ORGANS 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 maxilla, 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 Arca, 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 Kdlliker’s first class, ‘ echte 
Hirnaugen.’ It is indubitable that the nervous elements of the 
