X 



No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 20/ 



the eye shows plainly that the epidermis passes in to surround 

 the lens, as in Fig. 70. 



Actual sections show cell walls also round about the pigment 

 cup ; later the eye sinks away from the surface and appears in 

 section much as in Fig. 71. Here again a nucleus is seen in the 

 lens ; this may be explained in the way suggested for the eyes fig- 

 ured by Salensky. Yet it is to be observed that Meyer speaks 

 of the eyes as at first "bestehend aus je einer am proximalen 

 Theile ihres Peripherie mit rothbraunem pigment angefullten 

 Zelle " surrounded by a many-celled thickening of the ectoblast. 

 So he would regard the nucleus as really in the clear part of the 

 cell, not peripheral to the pigment, as I have maintained in this 

 paper. 



After these two eyes have moved away from the surface with 

 the brain, they become bilobed, suggesting to Meyer that they 

 subsequently divide to give rise to the groups of eyes subse- 

 quently present. 



Kleinenberg (I8) shows the larval eyes of a Phyllodoce in sec- 

 tion, resembling Fig. 21 save that cell walls and rods are not 

 present. 



The most detailed, the only detailed, account of the forma- 

 tion of the annelid eye is that given by Kleinenberg (18) for 

 certain Alciopidae. 



These remarkably large and complex eyes appear to be formed 

 in an even more remarkable manner, which opposes a strong 

 barrier to the extension of the conception advanced in the 

 present paper to include all the eyes of Polychaetae. 



In these Alciopidae the first appearance of the retina is a 

 solid ectoblastic thickening, one for each eye. These become 

 pinched off from the surface, without the appearance of any 

 invagination ; by a separation of the cells of this solid mass, a 

 central cavity is formed, in which a lens subsequently appears, 

 being secreted by the surrounding cells into the closed central 

 cavity. This retinal vesicle establishes a close connection with 

 the brain, even gives off some of its cells to the brain, but 

 later this connection is reduced to the stalk forming the optic 

 nerve. The part of the vesicle near the epidermis becomes 

 thin, and forms the inner cornea, while the rest remains as 

 the retina. The lens is from the first inside a closed sac ; is 

 at first granular, then homogeneous. It does not increase as 



