MUSEUM OF COMPARATIVE ZOOLOGY. 



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two ; cone cells, four ; proximal retinular cells, eight, one of which is 



rudimentary; distal retinular cells, two ; accessory cells, mesodermic (1) 

 in origin, often present. 



Table of Ommatidial Formula;. 



I have now concluded my account of the structure of the omraatidia 

 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 formulae 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 ^r. (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 CoMPOSixioy of the Ommatidian 

 Crustaceans. 



A few features in the table require explanation. Among the number 

 of cells recorded for the Estherid^e, the figure within the parenthesis 



