Oü the Visual Organs in Lamellibranchiata. 457 



believe to be the correct one and I am inclined to consider that the de- 

 finite described shape is due to contraction from the use of reagents ; 

 and then Hickson says that when removed, in the fresh condition, it is 

 not elliptical as in hardened sections , which he explains as being due 

 to relaxation of its ligaments. 



From the careful investigations of Hensen and preparations of my 

 own I am led to believe that the lens fills up the whole space and is 

 not the elliptical body figured and described by Hickson. 



Posterior to the lens is the retina, which extends across the 

 middle of the eye, perpendicularly to the axis of the eye and stalk. It 

 is separated from the posterior part of the eye by a thick layer of red- 

 dish pigment. According to Hickson the retina is raade up of five 

 layers and to Hensen only of three. I do not consider it necessary to 

 go into details concerniug this part , as it diflfers somewhat in form in 

 different species. 



Posterior to the retina is the tapetum, which gives rise to the 

 metallic lustre so striking in the living eye. Posterior to the tapetum 

 and filling up the cavity between it and the posterior wall of the eye- 

 ball is a mass of red fluid pigment consisting of «a number of bright red 

 granules«. A layer of cells according to Hensen forms the posterior 

 part of the eye, the existence of which is denied by Hickson. I have 

 preparations which demonstrate the existence of this layer and the 

 distinct cells and nuclei beyond all shadow of doubt; in my case how- 

 ever the cells are by no means as large nor as tali as those represented 

 by Hensen, and are not pigmented. 



The nervo US supply of this organ is very complicated ; two 

 nerve branches entering at two different points. The nerve comes from 

 the circumpallial nerve , as do nerves passing to the tentacles between 

 which the eye is situated. The branch which passes to the organ in 

 question divides into two branches , one passing into the base of the 

 ball called by Hickson the complementary nerve and the other he calls 

 the retinal, which passes up and enters the side of the ball and is 

 distributed to the anterior surface of the retina. 



The question as to the function of this organ is one of considerable 

 interest, Hickson states that a few experiments have been made on 

 this subject, concerning the visual power of this animai, and he says : 

 »It is very doubtful whether they (the so-called eyes)are of much value to 

 the animai in avoiding its enemies. The most reasonable theory of their 

 function seems to be that when on the ebbing of the tide , a probability 

 arises that they will be left high and dry on the shore, they can appre- 



