OCT 18 1899 



No. 6. — The Photomechanical Changes in the Retinal Pigment of 

 Gammarus. 1 By G. H. Pakker. 



The following studies were made with the intention of comparing the 

 photomechanical changes in the retina of one of the simpler crustaceans 

 with those already known in the decapods. In this respect, very few of 

 the simpler forms have been studied, and almost all that has been done 

 is contained in a single paper by Szczawinska ('91). This paper appeared 

 shortly before Exner's ('91) well known monograph, in which the pho- 

 tomechanical changes of the compound eye first received a consistent 

 physiological interpretation, and consequently it does not touch several 

 important questions raised by Exner's work. 



The lower crustaceans studied by Szczawinska were Gammarus, Phro- 

 nima, and Branchipus, and of these, judging from the figures given, 

 Gammarus has the most pronounced photomechanical changes. I have 

 therefore studied the common American form, Gammarus ornatus Milne- 

 Edwards. Vigorous individuals of this species were kept, some in the 

 dark and some in the light, for six hours, and were then killed by being 

 momentarily immersed in hot water (85° C). They were afterwards cut 

 in sections, and their eyes studied and compared. 



The structure of the eye in Gammarus ornatus has already been de- 

 scribed (cf. Parker, '91; p. 68), and agrees almost exactly with that of 

 G. pulex as given by Carriere ('85, p. 156). Szczawinska's ('91, p. 534) 

 description of the eye in G. roeselii is so different from the accounts 

 given for the other two species that I am persuaded it must be in part 

 erroneous. 



In G. ornatus the corneal cuticula (Fig. 1, crn.), which is not facetted, 

 is covered internally by a corneal hypodermis (cl. crn.) the cells of which 

 are not regularly grouped with reference to the ommatidia. The axis of 

 each ommatidiurh is occupied distally by a two-celled cone (con.) and 

 proximally by a slender rhabdome (rhb.). These two structures are 

 entirely distinct from each other and not continuous, as described by 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology at Harvard College. E. L. Mark, Director. No. C. 

 vol. xxxv. — no. G. * 



