456 Benjamin Sharp 



anatomy and histology of this complex organ has been well worked 

 out. In passing it is stränge to note that although many authors liave 

 stated that other forms possessed well developed eyes, Pecten have 

 always been taken for study (except by Will and Siebold) and no 

 mention of tbe anatomy of the eyes of other forms given, except that 

 they were similar organs. 



The following species of Pecten have been studied ; P. Jacohaeus^ 

 byPoLi, Grube, Krohn, Hensen and Hickson; P. opercularis by 

 Grube and Hickson ; P. varius by Grube, and P. maximus by Duver- 

 NOY, Keferstein and Hickson. The species studied by Garner is not 

 given, but is probably P. maximus. Will and Siebold's species are 

 not given. 



As the anatomy and histology of this so-called eye are so well 

 known , I will only give a brief account of it ; and I may add the spe- 

 cies P. hastatus and P. mageUani to the number examined. 



The eyeball is placed on a stalk or pedicle, which is covered by a 

 pigmented layer of epithelium , which extends into the eyeball as far 

 as the edge of the so-called cornea ; bere the epithelial layer looses its 

 pigment and becomes transparent and forms the anterior part of the 

 ball ; Hensen did not find this epithelial layer, but thought it might be 

 present. From the observations of Hickson and Schmidt and my own 

 preparations I do not doubt the existence of it. 



The cells of the pigmented epithelium have a stränge form and 

 are worthy to be taken up and considered in detail. They bear a dose 

 resemblance to the pigmented cells that I have described for Ostrea. 

 Two thirds of their length is filled with pigment (fig. 15 a and b) and 

 Hensen says that from some of his preparations he thought that they 

 might possess cilia (fig. 15 5). He further adds that the cells give the 

 Impression of being organs for the perception of light. My preparations 

 show similar cells, although no evidenees of cilia are present. It may 

 be that I have not handled the object with sufficient care. 



The eye is made up of , anteriorly the cornea, consisting of the 

 transparent epithelium resting on a hyaline membrane which is a thin 

 contiuuation of the connective tissues of the stalk. The lens which 

 fiUs the space immediately behind the cornea is large and made up of 

 nucleated cells. The lens is considered to be a spherical body by both 

 Krohn and Keferstein, while Hickson considers it elliptical. Hensen 

 considers the cells makiug up the so-called lens to fili up the space 

 between the retina and the anterior wall of the eye, making the shape 

 of an irregulär biconvex form. This latter statement I am inclined to 



