2 
G. H. Parker 
to a Parker Fellowship from Harvard University, Cambridge, U. S. A., 
a privilege that enabled me to enjoy the advantages of European 
travel and study. My investigations were carried on at several Con- 
tinental universities. Düring the winter Semester of 1891—92 I 
worked in the Zoological Laboratory at Leipzig under the direction of 
Professor Leückart. The following summer Semester was spent in 
Berlin at the laboratory of Professor F. E. Schulze. The succeeding 
winter Semester was passed at Freiburg in Baden, where most of 
mywork wasdone under the supervision of Professor Weismaxx, though, 
through the kindness of Professor von Kries, I also had the oppor- 
tunity of making some experiments in the Physiological Laboratory 
of that university. From Freiburg I went to the Zoological Station at 
Naples, where, by the courtesy of Professor Dohrn, I had the privi- 
lege of occupying a table for some three months. The encourage- 
ment to Investigation and the opportunities ofiPered at these institu- 
tions are too well-known to require comment, and the kiudliness and 
personal attention with which, as a Student, I was invariably received 
make it a pleasant duty for me to acknowledge my indebtedness to 
the directors and assistants under whom I worked. I am also under 
obligations to Dr. Otto vom Kath of Freiburg for suggestions and 
help in obtaining much necessary material, as well as to Signor 
Salvatore lo Bianco for his untiring efforts in procuring for me 
many interesting and important crustaceans. Although my studies 
were completed at Naples and the manuscript of the present paper 
was in part prepared there, I was forced to defer the completion of 
it tili my return to America. 
2. Materials and Methods. 
The two species of crayfish that I studied, were Asfacus fluvia- 
tilis Fahr., and Astacus leptodactylus Esch. The optic organs in 
these two animals are essentially similar, and the following account 
is believed to refer with equal accuracy to either species. In one 
respect, however, I have limited my observations ; to avoid the possi- 
bility of error resultiug from slight specific differences, I have taken all 
enumerations of nerve fibres, retinal elemeuts, etc. from one species, 
Astacus ßumatilis, Although my work has been done for the most 
part on the two species of crayfish mentioned, I have also studied, 
for the sake of comparison, representative species of the chief groups 
of crustaceans. 
