60 BULLETIN OF THE 
capable of motion, and in the Estheridz they are, if at all, only slightly 
movable. In the Cladocera, where the second type probably reaches its 
ereatest differentiation, the retina is remarkable for the freedom of its 
motion. 
In the third type the retina originates from thickened hypodermis, 
which subsequently separates into two layers, the corneal hypodermis 
and the retina proper (a layer of cones and retinule). This separation 
is accomplished either by the formation of a corneo-conal membrane, as 
in Amphipods, or by what I believe to be an actual withdrawal of the 
retina proper from contact with the hypodermis, as in Copepods. Only 
in the representatives of the extreme modification of this type, the Cope- 
pods, are the eyes movable. 
The course of development taken by each of the three types very 
clearly indicates their mutual relations. Evidently the first type is a 
primitive one, and since the first steps in the development of the second 
and third reproduce the permanent condition of the first, these two may 
therefore be considered derivatives from the first. It is interesting to 
observe that in the simpler condition of each type the retina is fixed, 
whereas in the more differentiated form it has become movable. The 
sinking of the retina into the deeper parts of the body, as represented in 
the second and third types, may have been induced by the protection 
thus obtained for the eye. After the three types were differentiated, 
each one seems to have been modified in a special way to give rise to a 
movable retina. 
ARRANGEMENT OF THE OMMATIDIA. 
The ommatidia in the retinas of some Crustaceans are so few in num- 
ber that they can scarcely be said to be grouped according to any system. 
Where they are numerous, however, they are arranged upon one or the 
other of two plans. These may be designated the hexagonal and tetrago- 
nal plans of arrangement. In the hexagonal plan the imaginary outline 
of the transverse section of an ommatidium is a hexagon, and each 
ommatidium, excepting those on the edge of the retina, is surrounded by 
six others. In the tetragonal arrangement the ideal transverse section 
of an ommatidium is a square. Each of the four sides of this square 
is occupied by one of the four faces of an adjoining ommatidium. 
The arrangement of the ommatidia can usually be determined by 
a careful inspection of the external surface of the eye; this determina- 
tion is considerably facilitated by the presence of a facetted cuticula. 
Sometimes the form of a single facet is sufficient to indicate the plan of 
