No. 6. — Photomechanical Changes in the Eetinal Pigment Cells of 

 Palcemonetes, and their Relation to the Ce7itral Nervous System. 

 By G. H. Parkek.1 



Contents. 



Introduction 275 



Structure of the Eye in Palaemonetes 276 

 Photomeclianical Changes in Nor- 

 mal Retiua 278 



1. Methods 278 



2. General Changes in Retina . 279 



3. Changes in Proximal Retinu- 



lar Cells 279 



4. Changes in Accessory Cells . 281 



5. Changes in Distal Retinular 



Cells 284 



Page 

 6. Summary of Changes in Nor- 

 mal Retina 288 



Sympathetic Photomechanical 



Changes 289 



Localized Photomeclianical Changes 290 

 Photomechanical Clianges in Ex- 

 cised Eyes and Retinas . . . 291 



General Summary 295 



Note 297 



Papers Cited 299 



Explanation of Plate. 



Introduction. 



The present paper is a record of a series of experiments on the photo- 

 mechanical changes in the pigment cells of the retina in Palcemonetes 

 vulgaris Stimp. This species is especially favorable for such work, 

 since its retina exhibits in a marked degree all the kinds of pigment 

 changes that liave thus far been observed in the eyes of crustaceans. 



Those that have worked upon this subject have, in the main, fol- 

 lowed in the lines laid down by Boll, Engelmann, and others in their 

 studies on the eyes of vertebrates. Although the pigment changes in 

 vertebrates are relatively simple, they are, even now, far from being 

 satisfactorily understood, and it is therefore not surprising that in 

 the arthropods, where the pigment changes of the retina are probably 

 more complex than in any other group of animals, much still remains 

 to be done. There has been a tendency, moreover, among some of 

 those that have studied such phenomena, to generahze on observations 

 taken from eyes of totally different types, such as the compound eyes 

 of insects and the simple eyes of arachnids ; and this tendency, though 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology at Harvard College, E. L. Mark, Director, No. LXXVI. 



VOL. XXX. NO. 6. 1 



