276 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



in a measure justifiable, has led, I believe, to a want of attention to 

 the characteristic difterences of the pigment changes in each given type 

 of eye, a matter that, in my opinion, lies at the foundation of any 

 satisfactory understanding of these changes. What is most needed at 

 present, therefore, seems to be a thorough and exhaustive study of the 

 pigment changes of each of the more important types, rather than an 

 inspection, necessarily more or less superficial, of the various arthropod 

 eyes that have not as yet been examined. The following studies have 

 been made with the hope that they would contribute in this respect to 

 a more complete understanding of the pigment changes in the com- 

 pound eye, especially in crustaceans. 



The earliest paper on the pigment changes in compound eyes, so far 

 as I am aware, was published in 1889 by Exner ('89), and contains in 

 a condensed form the essential peculiarities of the pigment changes 

 in the compound eyes of cei'tain insects. In 1890 Stefanowska ('90) 

 published an account in which this subject was again considered, but 

 with a wider range of material. In the next year three contributions 

 appeared: Exner's ('91) bi'illiant and imi^ortant essay on the physi- 

 ology of compound eyes, of which his former publication had been in 

 the nature of a partial preliminary notice ; Szczawinska's ('91) article 

 on the pigment changes in the eyes of crustaceans and arachnids ; and 

 Herrick's ('91) account of similar changes in the eyes of Paltemonetes, 

 contained in his monograph on the development of Alpheus. Three 

 years later Kiesel ('94) described some very noteworthy observations on 

 the pigment changes in the eyes of insects. The following year the writer 

 (Parker, '95) published, in connection with other matters, an account 

 of the retinal pigment changes in Astacus, and a preliminary state- 

 ment of the results given in full in this paper was published last year 

 (Parker, '96). These, I believe, are all the publications in which the 

 questions here raised have been considered. Critical comments on their 

 contents will be found in the following pages. 



Structure of the Eye in Pal^monetes. 



Before describing the pigment ahanges in the retina of Palsemonetes, 

 it will be necessary to outline briefly the structure of the eye in this 

 animal. The eye may be said to be that portion of the optic appai-atus 

 contained in the optic stalk. It consists of a retina, at the distal end 

 of the stalk, and a series of four optic ganglia, which extend through 

 the axial portion of the stalk. The retina is connected with the first 



