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 anatomical 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 celh, — 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 

 ommatidiuin consists of a cluster of cells regularly arranged around a 



