46 BULLETIN OF THE 
which I spent there in the summer of 1889, and included much of 
the material which I used in studying the eyes of Decapods. For the 
opportunities of collecting, both at Newport and Wood’s Holl, I am 
indebted to Dr. Alexander Agassiz. I also desire to express my thanks 
to Prof. M. McDonald, the United States Commissioner of Fish and 
Fisheries, for many courtesies shown me while at the government 
station at Wood’s Holl. 
Essentially the same methods as those which I used in investigating 
the eyes in the lobster were employed in studying the eyes in other 
Crustaceans. - As these methods have been described at some length in 
my paper on the lobster’s eye (Parker, ’90%, pp. 3, 4), further mention 
of them in this connection is unnecessary. 
Before proceeding to an account of the eyes in Crustaceans, a few 
statements should be made concerning the use of terms. In the fol- 
lowing anatomieal descriptions, I have very generally adhered to the 
older and more established terms. It must be admitted that some of 
these, on account of their derivation, are not entirely satisfactory, but 
because of their general acceptance I have chosen to retain them rather 
than to attempt to replace them by new ones. 
The term retinula, the use of which varies with different writers, was 
introduced by Grenacher (’77, p. 17), who employed it to designate the 
rhabdome and the group of cells by which this structure is surrounded, 
Subsequently, Patten (’86, p. 544) used the same term as a name for 
a single cell of the group to which Grenacher gave the name retinula, 
In my paper on the eyes of the lobster I followed Patten’s usage, but 
in the present paper I have decided to employ the term as originally 
defined by Grenacher, and to designate the individual cells in the 
retinula as retinular cells, —a translation of the term already used for 
this purpose in many German publications. 
The greater part of the present paper is taken up with descriptions 
of the eyes in different Crustaceans. The amount of detail thus col- 
lected is considerable, and might appear at first sight to include many 
unimportant particulars; but the number of observations recorded is 
justifiable, I believe, on the ground that the majority of them bear 
more or less directly upon the solution of the principal question dealt 
with in the paper. 
The following statements will make clear the character of this ques- 
tion. It is now well recognized that the retina in compound eyes is 
composed of a number of similar units or ommatidia, and that each 
ommatidium consists of a cluster of cells regularly arranged around a 
