478 ONCHIDIUM. 



cells (fig. 279). When the inner segment of the lens is far 

 advanced towards its complete formation pigment becomes 

 deposited in the anterior part of the retina, and a layer of rods 

 grows out from the surface turned towards the cavity of the 

 optic vesicle (fig. 280 A, st). At a slightly later stage the retina 

 becomes divided into two layers (Bobretzky), a thicker anterior 

 layer, and a thinner posterior layer (fig. 280, rt and rf}. The 

 former is composed of two strata, (i) the rods and (2) a stratum 

 with numerous rows of nuclei which becomes in the adult the 

 granular layer with its pigment. The posterior layer gives rise 

 to the cellular part of the posterior division of the retina, while 

 layers of connective tissue around it give rise to the connective 

 tissue of this portion of the retina (layer 6 in the scheme on 

 p. 474). The nervous layer is derived from the optic ganglion 

 which attaches itself to the inner side of the connective tissue layer. 



The greater part of the choroid is formed from the mesoblast 

 adjoining the retina, but the epithelium covering its outer wall 

 is of epiblastic origin. 



It is difficult to decide from development whether the Mollus- 

 can eyes, so far dealt with, originated in the first instance part 

 passu with the supra-cesophageal ganglia or independently at a 

 later period. On purely a priori ground I should be inclined to 

 adopt the former alternative. 



In addition to the above eyes there occur amongst Mollusca highly 

 complicated eyes, of a very different kind, in two widely separated groups, 

 viz. certain species of a genus of slug (Onchidium), and certain Lamelli- 

 branchiata. These eyes, though they have no doubt been evolved indepen- 

 dently of each other, present certain remarkable points of agreement. In 

 both of them the rods of the retina are turned away from the surface, and 

 the nerve-fibres are placed, as in the Vertebrate eye, on the side of the retina 

 which faces outwards. 



The peculiar eyes of Onchidium, investigated by Semper 1 , are scattered 

 on the dorsal surface, there being normal eyes in the usual situation on the 

 head. The eyes on the dorsal surface are formed of a cornea, a lens 

 composed of i 7 cells, and a retina surrounded by pigment ; which is 

 perforated in the centre by an optic nerve, the retinal elements being in the 

 inverted position above mentioned. 



The development of these eyes has been somewhat imperfectly studied 

 in the adult, in which they continue to be formed anew. They arise by a 



1 Ueber Sehorgane von Typus d. Wirbdthieraugen, etc., Wiesbaden, 1877, anf l 

 Archiv f. mikr. Anat. Vol. xiv. pp. 118 122. 



