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 resj.ects 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 

 maximus, but I have also had the opportunity of making 

 sections of and studying the eyes of two other species, 

 Pecten jacohceus 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.jacohaus. 



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 lactile 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. Cliatin, ' Bulletin de la Societe Philomatique.' Paris, 1877. . 



