280 K. KISHINOUYE. 



side exposed to the li^ht. 



The eye can easily: he found without a microscope as a black spot 

 on the si|)honal side of the tip of a tentacle, opposite to the position of 

 the eye of Cnniium efhile, which is '' on the shell side of the mantle." * 

 Along the upper, posterior f side of the eye, the epithelium is raised 

 into a triangular screen (figs. '2. 6, 7, 9, .s.). At one time I thought 

 that this screen is the tip of the tentacle, but a closer examination 

 showed that it is raised secondarily, probably io ])rotect the eye, and 

 that the true tip is occupied by the eye. 



In the eye i^f Cnrdium muticum we find all the essential ])arts 

 of an eye. Its structure resembles on the whole that of the eyes of 

 Fecten. Spondyiius^ and Cardivm edide ; but ditlers in S(jme essential 

 points, since it is ditHcult to find the liomology of some constituent parts. 



The cornea (fig. 2. <;.) consists of thin ])a veinent cells, while that 

 of other molluscs consistSigenerally of columnar cells. The general 

 epithelium ber-omes gradually thin as it ap])roaches the lens and forms 

 the thin cornea over it. The corneal cells are colourless, transparent, 

 and polygonal in outline. The convexity of the cornea is great, its 

 external surface being almost hemispherical. 



The lens (fig. 2, /.) is large and consists of a great number of 

 cells. Instead of the usual biconvex, flat form the lens of Cardium 

 imdicwn is ovoid. Its longer axis is parallel to the o])tic axis and 

 its broader end is directed below. It is more or less constricted at 

 the middle' ]])art. The cells composing the lens are large, colourless 

 and compressed in the direction of the optic axis, the degree of 

 compression being greater nearer the cornea. In the median lon- 



* Patten— Eyes of Molluscs and Arthropods. Mittheil, aus der Zool. Station zu Neapel. 

 188G. 



t For the sake of convenience, the word upper is used to designate the distal end of the 

 tentacle, and the word anterior that side of the tentacle turned towards the sijihonal openings. 

 'I'he words lower and posterior naturally indicate the opposites of the above worda. 



