MUSEUM OF CO.MPAPwVTIVE ZOOLOGY. 67 



The cells of the corneal hy[)odermis are usually arranged in a very 

 thin layer, and constitute tlie most superficial tissue in the I'etina. 

 They either present no definite arrangement, as in Amphipods, or tiiey 

 are regularly grouped in pairs, one pair for each ommatidium, as in 

 the majority of Crustaceans. On their external faces they produce the 

 corneal cutlcula. This is unfacetted in those Crustaceans in which the 

 corneal cells are not regularly arranged and facetted when they ai-e 

 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 tlie rhabdome. The cone 

 occupies the axis of the distal portion of the onmiatidium. 



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 tliese 

 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 

 larvge 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 



