124 BULLETIN OF THE 



number of their elements, it would naturally follow that those com- 

 posed of the fewest cells would more nearly resemble the ancestral type 

 than those which consist of many cells. On the other hand, if the sup- 

 pression of cells were the only means employed in modifying structure, 

 the ommatidia containing the greatest number of elements would most 

 nearly approach the primitive type. Since, as I believe, both means 

 are employed in the Crustacea, the determination of the structure of 

 the ancestral ommatidium is evidently a difficult problem. Perhaps 

 the most satisfactory way of attempting its solution is to consider sep- 

 arately the different categories of cells which enter into the formation of 

 an ommatidium, and, after reviewing the conditions presented by each 

 in different Crustaceans, to determine, if possible, which of these condi- 

 tions is the most primitive. The conclusions thus arrived at concerning 

 each kind of cell will afford the necessary grounds for the construction 

 of an hypothetical formula of the ancestral ommatidium. Although it 

 is not necessary that this ommatidium should be represented in any liv- 

 ing Crustacean, for the ommatidia in all these may have suffered modifi- 

 cation, yet it is possible that a representative of it may still exist. 



Turning now to the consideration of the different groups of cells, we 

 find that the corneal hypodermia presents two conditions; one in which 

 its cells are not regularly arranged, and another in which they are 

 grouped in pairs, each pair lying at the distal end of an ommatidium. 

 The latter condition is characteristic of the Decapods, Schizopods, Sto- 

 matopods, NebalisB, Isopods, and some Branchiopods; the former, so far 

 as is known, occurs in the Amphipods, the Branchiura, and in some 

 Branchiopods (Limnadia and some species of Branchipus). In view of 

 the fact that the corneal hypodermis is a part of the retina which re- 

 tains the function of the general hypodermis but slightly modified, and 

 that in the latter the cells do not present a regular arrangement, it 

 is probable that a corneal hypodermis in which the cells are not regu- 

 larly arranged is of a more primitive character than one in which they 

 are definitely grouped. 



The number of cells in the individual cones of Crustaceans varies 

 from two to five. Cones composed of two cells occur in Eucopepoda, 

 Amphipods, Isopods, and Schizopods; cones of three cells are present 

 only exceptionally in Isopods ; cones of four cells are found in the 

 Decapods, Stomatopods, Xebalise, Branchiura, and some Branchiopods ; 

 cones of five cells characterize the Cladocera and some Branchiopods. 

 I have already given reasons for regarding the cones composed of three 

 cells as having been derived from those containing two, and cones com- 



