50 . \V. J. DAKIN. 



more confusing series of results had been added to the already 

 existing multiplicity. 



I determined therefore to make a complete study of the 

 histology of the eye. The privilege of occupying the British 

 Association Table at Naples enabled me to carry out this 

 investigation on a species previously examined by most 

 writers on the eye — Pec ten jacobaeus — and this was com- 

 pleted by a considerable stay at the Port Erin Biological 

 Station. The results have been the discovery of several new 

 points, the confirmation and refutation of many discoveries of 

 different workers, and I hope the complete elucidation of the 

 structure of the retina. It has been due to the frequency of 

 occurrence of artefacts and the difficult histological work 

 required for such complicated organs that the structure of 

 these eyes has remained so long a puzzle. 



By the use, however, of numerous methods it has been 

 possible to eliminate to the greatest extent the artefacts, and 

 incidentally the trial of so many fixatives, etc., has enabled 

 me to obtain practically all the appearances seen and figured 

 by the various investigators. 



The account of the structure will be given at some length, 

 since a comparison of the various views is necessary, and, 

 with the exception of Hickson's and Patten's papers very 

 little has appeared in English. I am indebted to the British 

 Association for permission to use their table at the Zoological 

 Station of Naples, and also to the staff of that well-known 

 institution. My thanks are also due to Professor Herdman 

 and to the Curator of the Port Erin Biological Station for the 

 trouble taken in supplying me with material and apparatus 

 for carrying out detailed work at the latter place, and to Pro- 

 fessor Drew, of Maine, for specimens of P. tenuicostatus. 



History. 



Only the history of the references to the Pecten eye before 

 and including the fundamental paper of Hensen will be given 

 in this section, since the other works will be discussed more 



