ORGANS OF VISION. 479 



differentiation of the epidermis at the end of a papilla. At first a few 

 glandular 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 

 surrounded 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 papillce. 



The eyes of Pecten and Spondylus 1 are placed on short stalks at the 

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

 processes 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 Vertebrate 

 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 Echinodermata is too 

 scanty to be of any general interest. 



Chaetopoda. Amongst the Chaetopoda the cephalic eyes of Alciope 

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

 These are provided with a large cuticular lens (/), 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 Polyophthalmus, 

 and have probably been evolved from the more indifferent type of sense- 

 organ found by Eisig in the allied Capitellidas. 



Chaetognatha 2 . The paired cephalic eyes of Sagitta are spherical 

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

 pigment with three lenses partially imbedded in it. The outer covering of 

 the eye is the retina, which is mainly composed of rod-bearing cells ; the 

 rods being placed in contact with the outer surface of each of the lenses. In 

 the presence of three lenses the eye of Sagitta approaches in some respects 

 the eye of the Arthropoda. 



Arthropodan eye. A satisfactory elucidation of the phylo- 

 geny of Arthropodan eyes has not yet been given. 



All the types of eyes found in the group (with exception of 



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

 Micr. Science, Vol. xx. 1 880. 



2 O. Hertwig. " Die Chaetognathen." Jenaischc Zcitschrift, Vol. XTV. 1880. 



