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MUSEUM OF COMPARATIVE ZOOLOGY. 67 
The cells of the corneal hypodermis are usually arranged in a very 
thin layer, and constitute the most superficial tissue in the retina, 
They either present no definite arrangement, as in Amphipods, or they 
are regularly grouped in pairs, one pair for each ommatidium, as in 
the majority of Crustaceans. On their external faces they produce the 
corneal cuticula. This is unfacetted in those Crustaceans in which the 
corneal cells are not regularly arranged and facetted when they are 
grouped in pairs. 
The cone cells in each ommatidium are united to form the cone, a 
transparent body which extends from the corneal hypodermis proximally 
through the ommatidium at least as far as the rhabdome. The cone 
occupies the axis of the distal portion of the ommatidium, 
The proximal retinular cells are usually limited to the proximal por- 
tion of the ommatidium. They are definitely arranged around the 
axial structure of that region, the rhabdome, and together with it form 
a single body, the retinula. The optic nerve fibres terminate -in the 
proximal retinular cells. 
The distal retinular cells are present in only the more differentiated 
ommatidia. They are two in number, and invest the sides of the cone 
distal to the plane at which this structure emerges from the retinula. 
When distal cells are present, the remaining cells of the retinula will be 
distinguished as proximal cells; when the distal cells are wanting, the 
other cells will be called simply retinular cells. 
The accessory cells fill the space between the elements of an omma- 
tidium, or between separate ommatidia, Their number is apparently 
inconstant, and they present a variety of forms. They may or may 
not contain pigment. Depending upon their source, two kinds can be 
distinguished, ectodermic and mesodermic. 
In describing the ommatidia, I shall consider them according to the 
groups of Crustaceans in which they occur. Under each group the 
elements comprising the ommatidium will be described in the order 
in which they have just been mentioned. 
My object in the following account is to determine, as far as possible, 
what the different kinds of ommatidial types are, and to define these 
types by a brief statement of the number and kinds of cells which char- 
acterize them. 
Compound eyes are known to occur in some Ostracods, and in the 
larve of some Cirripeds, but their histological structure, I believe, has 
never been studied. I am therefore compelled to dismiss these two 
groups without further comment, and proceed with the description of 
