394 ONCH1DIUM AND PEC TEN. 



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 othei-, 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 out wards. 



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 siirface are formed of a cornea, a lens 

 composed of 1 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 dif- 

 ferentiation of the epidermis at the end of a papilla. At first a few glan- 

 dular cells appear in the epidermis in the situation where an eye is about 

 to be formed. Then, by a further process of growth, an irregular mass of 

 epidermic cells becomes developed, which pushes the glandular cells to one 

 side, and constitutes the rudiment of the eye. This mass, becoming sur- 

 rounded by pigment, unites with the optic nerve, and its cells then differ- 

 entiate themselves, in situ, into the various elements of the eye. No 

 explanation is offered by Semper of the inverted position of the rods, nor 

 is any suggested by his account of the development. As pointed out by 

 Semper these eyes are no doubt modifications of the sensory epithelium 

 of the papilla?. 



The eyes of Pecten and Spondylus 2 are placed on short stalks at the 

 edge of the mantle, and are probably modifications of the tentacular pro- 

 cesses of the mantle edge. They are provided with a cornea, a cellular 

 lens, a vitreous chamber, and a retina. The retinal elements are inverted, 

 and the optic nerve passes in at the side, but occupies, in reference to its 

 ramifications, the same relative situation as the optic nerve in the Verte- 

 brate eye. The development has unfortunately not yet been studied. 



Our knowledge of the structure or still more of the development of the 

 organ of vision of the Platyelminthes, Rotifera, and Echinoderniata is too 

 scanty to be of any general interest. 



ChSBtopoda. Amongst the Chsetopoda the cephalic eyes of Alciope 

 (fig. 281) have been adequately investigated as to their anatomy by Greetf. 

 These are provided with a large cuticular lens (I), separated from the retina 

 by a wide cavity containing the vitreous humour. The retina is formed 

 of a single row of cells, with rods at their free extremities, continuous at 

 their opposite ends with nerve-fibres. The development of this eye has 

 not been worked out. Eyes not situated on the head are found in Polyoph- 

 thalmus, and have probably been evolved from the more indifferent type 

 of sense-organ found by Eisig in the allied Capitellidse. 



Chsetog'natha 3 . The paired cephalic eyes of Sagitta are spherical 

 bodies imbedded in the epidermis. They are formed of a central mass 



1 Ucber Sehorgane ran Typus d. Wirbelthieraugen, etc., Wiesbaden, 1877, and 

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



2 Vide Hensen (No. 364) and S. J. Hickson, " The Eye of Pecten," Quart. J. of 

 Micr. Science, Vol. xx. 1880. 



3 O. Hertwig. "Die Chaetopnathen." Jenaische Zeitsclmft, Vol. xiv. 1880. 



