I. INTRODUCTION. 



The structural elements of the eye of Pecten and its nerve distribution have 

 been described in great detail by Patten ('86) in his article "Eyes of Molluscs and 

 Arthropods," and before him by Hensen ('65) and by Hickson ('80). Since these 

 publications, however, improved methods for the impregnation of nerves have been 

 developed, with the aid of which more correct views and interpretations of structures 

 have been gained, leading not only to broader morphological comparisons and gener- 

 alizations, but also to the necessity of repeating the investigation of organs concern- 

 ing which there are differences of opinion. Among the latter is the eye of Pecten, 

 for the earlier descriptions of the retinal nerve-endings in this sense-organ have been 

 questioned. It became necessary, therefore, to subject this eye to a renewed study 

 with modern methods. 



The study of the nerve-endings in the eyes of some Molluscs which I began in 

 1897 under Dr. Mark at Radcliffe College was continued on marine forms, especially on 

 Pecten, during the summer of 1898 at the Marine Biological Laboratory at Wood's Hole, 

 Mass. Much material was examined and preserved for sections and for maceration. 

 Now after four years it is taken up for completion. Besides the Golgi and methylen- 

 blue, most of the later methods used in neurological work were employed. Of the 

 hardening fluids used for control study, Apathy's osmic sublimate, Niessing's platino- 

 osmic sublimate, and Vom Rath's picro-platin-osmic mixture gave good results, and iron 

 haematoxylin proved to be the best stain. Of the methylen-blue, a modification of 

 Bethe's'method proved most satisfactory. The animals were either injected with a .5% 

 solution of methylen-blue in Pecten-serum that was diluted with a 0.6% solution of 

 sodium chloride, and after ten hours their eyes were removed and hardened; or the 

 eyes were removed from an uninjected animal, put into a dilute solution of methylen- 

 blue in Pecten-serum for two hours, and then exposed in a fresh solution for two hours 

 longer. They were then hardened for two hours either in 10% ammonium molybdate, 

 or in ammonium molybdate containing hydrogen peroxide and hydrochloric acid, 

 or in 10% ammonium molybdate to which one-tenth its volume of 3% formol and 

 a drop of 2 % osmic acid to every 5 cubic centimetres of the solution were added. They 

 were washed in water for an hour, dehydrated, and finally put into oil of bergamot or 

 xylol. All solutions were kept cool by placing them near ice. Eosin dissolved in a 



