No. 3. — CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 

 OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD 

 COLLEGE, UNDER THE DIRECTION OF E. L. MARK, No. 174. 



The Eyes of certain Pulmonate Gfasteropods, with special 

 Reference to the Neurofibrillae in Limax maximus. 



By Grant Smith. 

 TABLE OF CONTENTS. 



PAGE 



Introduction 233 



Material and methods 237 



The morphology of the eye of Limax 241 

 The histology of the eyes of Limax 



maximus and Helix pomatia . . 245 

 Some observations on the eye of 



Planorbis 254 



PA3E 



Neurofibrillae in the eye of Limax . 255 



General discussions and conclusions 265 



Summary 277 



Bibliography 279 



Explanation of plates 283 



Introduction. 



The brilliant work of Apathy ('97) on neurofibrillae has not only led 

 to a modification of the neuron theory, but has also brought about a 

 renewal of interest in the structure of light-recipient organs and a revo- 

 lution in our conception of them. In molluscs, as in other invertebrates, 

 the discovery of the general form of the light-recipient structures — 

 which, for convenience, we may call rods — long remained the goal of 

 investigators. A concise statement of the main historical facts up to his 

 own time is given by Hilger ('84). I shall pass by the earlier works and 

 refer briefly to those which usher in a more precise histological treatment 

 of the retinal elements. 



The first considerable advance in a knowledge of the retina in gastero- 

 pods was obtained from studies on the eyes of Heteropoda. These eyes 

 were pointed to by Leuckart ('54, p. 32) as being peculiarly fitted to 

 serve in the solution of problems concerning the finer structure of 

 the light-percipient apparatus of animals in general. Leuckart himself 

 (pp. 27-34) gave an admirably clear account of the mutual relations of 

 optic nerve, pigment, and retinal elements in Pterotrachea, and described 

 in some detail the form, size, arrangement, and relations of the rods. In 

 the following year Gegenbaur ('55, p. 166) described somewhat more 

 precisely the retinal rods of the same animal, saying that they consisted 

 of viscid, homogeneous contents surrounded by a clear and delicate 



