MUSEUM OF COMPARATIVE ZOOLOGY. 115 
two; cone cells, four; proximal retinular cells, eight, one of which is 
rudimentary ; distal retinular cells, two ; accessory cells, mesodermic (?) 
in origin, often present. 
TABLE OF OMMATIDIAL FoRMULZ. 
I have now concluded my account of the structure of the ommatidia 
in Crustaceans, and for the purpose of presenting in a condensed form 
its more important features I have devised the following table. This 
consists of a series of ommatidial formule constructed upon the plan 
which I have described in the Introduction. The figures indicate the 
numbers of particular kinds of cells present in the ommatidium of a 
given group. The abbreviation pr. (present) marks the presence of any 
kind of cell when the number of that kind is not constant for different 
ommatidia in the same individual. 
TABLE SHOWING THE CELLULAR COMPOSITION OF THE OMMATIDIAN 
CRUSTACEANS. 

Retinular Cells. 
Cells of el ae pL Ps PET ee 
Corneal Differentiated. Accessory 
Groups of Crustaceans. Hy po- Undiffer- Cells, 
dermis. entiated.| Proxi- . 
aaare Distal. 
Amphipoda, - pr. (ect. 2) 
Branchiopodide and Apuside, 0 


Estherida, : 0 
Cladocera, i pr. (ect. 2) 

Copepoda: Pontella, ; pr. (ect. 2) 
“ 
Sapphirina, 
Argulus, f 2 
Tsopoda: Idotea, pr. (ect. 2) 
Porcellio, 2 pr. (ect. ?) 
Serolis, pr. (ect. ) 
Nebaliz, pr. (ect. 2) 
Schizopoda, pr. (mes. ?) 

Stomatopoda, pr. (mes. 2) 
pr. (mes. ?) 

A few features in the table require explanation. Among the number 
of cells recorded for the Estheride, the figure within the parenthesis 
