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 hypodermis 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, Nebaliz, 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, Nebaliz, 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- 
