MUSEUM OF COMPARATIVE ZOOLOGY. 73 



to this third kind, although, as must be remembered, Grenacher states 

 that there are only two such cells for each ommatidinm. 



These three kiuds of accessory pigiueut cells, with the possible excep- 

 tion of those which surround the retinula, occur in the lateral eyes of 

 Phronima (Carriere, '85, p. 1 64). 



Almost nothing is known about the source of the accessory pigment 

 cells in Amphipods. Those in Gammarus have no resemblance to the 

 loose mesodermic tissue which lies in the neighborhood of the eye, and 

 they are probably derived from the original ectodermic thickening which 

 gave rise to the retina. Although some of the accessory pigment cells 

 in Hyperia and Phronima have been called connective-tissue cells (Glaus, 

 '79, p. 125, Carriere, '85, p. 160), a name which might be taken to im- 

 ply that they have come from a mesodermic source, nothing is really 

 known about them which would be inconsistent with an ectodermic. 

 origin. 



From the foregoing account of the ommatidia in Amphipods the follow- 

 ing summary can be made: cells of the corneal hypodermis not definitely 

 arranged, from about nine to twelve, — possibly two to each ommatidium ; 

 cone cells, two ; retinular cells, five, — possibly in some cases four; ac- 

 cessory pigment cells (ectodermic]) present. Of these last there may be 

 only one kind, as in Gammarus and Talorchestia, or there may be three 

 kinds, as in Hyperia. 



Phyllopoda. 



The ommatidia in the eyes of Phyllopods present at least two struc- 

 tural types, one of which obtains in the Branchiopodidas and Apusidse, 

 the other in the Estheridaa and Cladocera. On account of the greater 

 convenience, the eyes in the Apusidse and Branchiopodida? will be con- 

 sidered first, then the eyes in the Estherida?, and finally those in the 

 Cladocera. 



Branchiopodidce and Apusitbe. — The ommatidia in these two families, 

 and especially in the Branchiopodidee, have been carefully studied by a 

 number of competent investigators ; their structure is consequently 

 well known. 



The material which I used in studying these eyes consisted of speci- 

 mens of Branchipus, probably B. vernalis, Verrill, which I had collected 

 in the neighborhood of Philadelphia, and which had been preserved 

 for some time in strong alcohol. Through the kindness of Dr. W. A. 

 Setchell, I was also able to examine a specimen of Apus lucasanus, 

 Packard. 



