276 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in a measure justifiable, has led, I believe, to a want of attention to 
the characteristic differences 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 Iam aware, was published in 1889 by Exner (789), and contains in 
a condensed form the essential peculiarities of the pigment changes 
in the compound eyes of certain 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) brilliant and important 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 Palamonetes, 
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. 
ÖTRUOTURE OF THE Eym IN PALZXMONETES. 
Before describing the pigment changes in the retina of Paloemonetes, 
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 apparatus 
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 
