52 THE AMERICAN MONTHLY [Apr. 



verging so as to enclose a spherical chamber, devoid of lens 

 and cornea in some instances, but having a minute hole 

 communicatiug with the outside and filled with sea-water 

 during life. 



The eyes of Pecten however originated not as pits or 

 depressions in the exterior membrane but as tentacles 

 and while in the cephalic eyes of Mollusks the fibers of 

 the optic nerve join the posterior nerve-end cells; in this 

 instance the optic nerve penetrates the capsule of the eye 

 and passes in front of the retinal body so that its fibers 

 are inserted into the anterior aspect of the rods as they 

 are in vertebrates. 



Again the lens in the eye of Pecten is not a product of 

 the cuticle as is the case in most Mollusks where the 

 closed cavity is wholly or partially filled with a refrac- 

 tive body, the lens being secreted from the walls, but is 

 a cellular structure which again corresponds with the 

 eyes of vertebrates. 



Thus we note several points of agreement in the eye of 

 this Mollusk with those' of higher organizations and by 

 carefully manipulating a portion of the mantle-skirt con- 

 taining the eyes we can demonstrate these facts for 

 ourselves. 



In order to study the eye, small portions containing 

 one or two eyes and not over two or three centimeters 

 thick should be cut from the mantle-skirt and properly 

 treated by fixing, hardening, embedding, cutting, 

 staining, etc. 



To show the retina the best results will be obtained by 

 fixing in a 1 per cent solution of Osmic Acid, although 

 excellent general results may be had after fixing in 40 

 per cent sol. Formaldehyd or a saturated aqueous solu- 

 tion of Mercuric Chloride. After fixing with Mercuric 

 Chloride the tissue stains beautifully with Borax-Car- 

 mine, but no staining whatever is required if the tissue 

 is fixed and hardened in Osmic Acid. 



