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 ommatidium. 
These three kinds of accessory pigment cells, with the possible excep- 
tion of those which surround the retinula, occur in the lateral eyes of 
Phronima (Carriere, ’85, p. 164). 
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 connectiye-tissue cells (Claus, 
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 Branchiopodide and Apuside, 
the other in the Estheride and Cladocera. On account of the greater 
convenience, the eyes in the Apusidze and Branchiopodide will be con- 
sidered first, then the eyes in the Estheride, and finally those in the 
Cladocera. 
Branchiopodide and Apuside. —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. 
