52 BULLETIN OF THE 
loses its connection with the exterior, and becomes reduced to a closed 
sac on the distal face of the retina. With the closure of the sac, the 
continuity of the retina with the superficial hypodermis becomes in- 
terrupted. 
In other Cladocera, especially the genera Sida and Daphnia, Grobben 
has found evidence to believe that the eyes are of essentially the same 
structure as in Moina. In a majority of the Cladocera the two com- 
pound eyes coalesce even more completely than in Limnadia. 
In the development of Moina, as the preceding description indicates, 
the eye passes through a phase which closely resembles the permanent 
condition in Limnadia, ‘he eye in the latter may therefore be inter- 
preted as representing a stage in the phylogeny of the eye in Moina. 
In accordance with the facts presented in the foregoing account, the 
second retinal type can be described as one in which the retiua does not 
retain its primitive external position, but sinks below the surface of the 
animal and becomes covered by a fold of the integument. The optic 
pocket thus formed may remain permanently open, as in the Apuside 
and Estheride, or may become closed and partially obliterated, as in 
the Cladocera. The right and left retinas either remain separated, as in 
the Apuside, or become closely approximated, as in the Estheride, or 
fused, as in the Cladocera. 
The minor modifications which this retinal type presents are not with- 
out importance. Bearing in mind the general statement that the com- 
pound eyes in Crustaceans are separate, paired, superficial structures, it 
is evident that the eyes in the Apuside, in which the retinas are sepa- 
rate and the optic pocket permanently open, depart only slightly from 
the primitive condition. In the Estheride, in which the two retinas 
are closely approximated, the eye is farther removed from the original 
type; but not so far as in the Cladocera, in which not only the two 
retinas are fused, but the optic pocket is closed and partially obliterated, 
thus entirely disconnecting the retina from the hypodermis. The three 
groups —the Apuside, the Estheridz, and the Cladocera— may con- 
sequently be taken to represent a series in the differentiation of the 
second retinal type. That this series is a natural one, and that it cul- 
minates in the Cladocera, is shown from the fact that in the develop- 
ment of Moina, and perhaps many other Cladocera, the eyes pass 
through stages which reproduce the essential features of the perma- 
nent condition in the Apnside and Estheride. 
In the THIRD RETINAL TYPE, as in the more differentiated form of 
the second, the retina is completely separated from the hypodermis. 
