THE EYE OF PECTEN. 5 



who have written on this subject. Krohn and Keferstein 

 believed it to be spherical. Hensen has figured it filling up 

 the space between the cornea and retina, and consequently 

 of an irregular bi-convex shape. 



It is difficult to see how a controversy on such a simple 

 subject could have arisen, unless it is because different 

 authors have examined different species, and described them 

 for the genus. 



As regards Pecten maximus,?^n examination of the fresh eye 

 has convinced me that in this species the lens is elliptical, the 

 major axis being parallel to the plane of the mantle. A section 

 of the eye made in a plane at right angles to the plane of the 

 mantle and the direction of its margin — that is^ the plane 

 which is most convenient for section-cutting, and the one 

 which is apparently usually adopted — would consequently 

 cause the lens to appear circular in section. In the dia- 

 grammatic representation of the eye (fig. 1) I have for con- 

 venience sake represented the lens as being at right angles 

 to the plane of the mantle in order that the true shape of 

 the lens may not be overlooked. 



A fresh examination of the lens, when teased out from the 

 rest of the eye, exhibits one or two interesting points. The 

 lens is not, as in most eyes, perfectly colourless, but possesses 

 a well-marked brown colouration, and a number of fine striae 

 may be seen running in the direction of the major axis. 

 The lens does not appear so perfectly elliptical in the fresh 

 condition as in certain sections 1 have made; it is drawn 

 out somewhat longitudinally, so as to he more like a double 

 cone than an ellipse. This is probably due to the lens being 

 released from the ligaments and connective-tissue pressures, 

 which cause it to retain its proper shape. 



Hensen says that the lens is very soft, and the cells are 

 light, polygonal, and nucleated. A careful examination 

 of the lens of P. maximus has led me to a very different con- 

 clusion. The lens seemed to be of exactly the same nature 

 as in the higher forms, and when teasing it out I found some 

 difficulty in holding it with a needle, as it slipped away from 

 under it when a slight pressure was exerted. As regards 

 the shape of the cells composing the lens, they are not all 

 polygonal, as would be inferred from Hensen^s remarks on 

 the subject. In the centre they are polygonal, but as they 

 approach the periphery they become more and more flat- 

 tened and elongated, until at the periphery they are strap- 

 shaped. They are nucleated. As Hensen, I could find no 

 membrane covering the lens, and no muscular fibres con- 

 nected with it; but in a few cases I have observed a liga- 



