444 
SYDNEY J, HICKSON. 
gave figures of the characters of any of the histological 
elements. But as the eye of Pecten forms only a very small 
part of the paper^ his figures and description are by no 
means complete^ and in many respects they are incorrect. 
Finally, J. Chatin^ has contributed two short papers, with- 
out figures, on this subject. 
Of the scanty literature Hensen’s paper is by far the 
most important, and he alone gives any good figures of 
sections of the eye, or of its elements ; the other observers 
give remarkably few figures, and consequently I have had, 
owing to an imperfect knowledge of the German language, 
some difficulty in making myself acquainted with the sub- 
stance of their papers. 
I have been encouraged in publishing the following re- 
searches chiefly by this scarcity of good figures, but also 
because I believe, and will give my reasons for believing, 
that these eyes deserve more mention than is usually made 
of them in our zoological text-books. 
My investigations were chiefly carried on upon Pecten 
maximuSj but I have also had the opportunity of making 
sections of and studying the eyes of two other species, 
Pecten Jacob ecus and Pecten opercularis. The eyes of these 
three species differ from one another in one or two not 
altogether unimportant particulars, and, as I shall after- 
wards point out, they form an interesting gradation, the 
points of difference between P. ^maximus and P. oper- 
cularis passing through intermediate stages in P.jacohceus. 
The eyes of Pecten maximus — are situated amidst a number 
of tentacles, which run all round the border of the mantle. 
These tentacles are capable of considerable movement, and 
frequently overhang the eyes and protect them from the 
light. The eyes themselves are situated upon short stalks, 
which resemble very closely the basal part of an ordinary 
tentacle. 
This similarity caused Duvernoy to name a tentacle a 
tactile pedicel, and an eye an ocular pedicel, thus to a cer- 
tain extent implying that they are morphologically homo- 
logous organs respectively modified for a tactile and an 
ocular function. This homology is justified by certain 
points in their anatomy, such as the course of the nerve and 
the arrangement of the muscular fibres, and I believe that 
when the development of these eyes is studied the homology 
will be still further confirmed. 
The border of the mantle which bears the tentacles and 
^ J. CliatiD, ‘Bulletin de la Societe Pliilomatique.’ Paris, 1877. 
