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 hjpodermis 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 Liumadia. 



In the development of Moina, as the preceding description indicates, 

 the eye passes through a phase which closely resembles the permanent 

 condition in Limnadia. The 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 retina 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 Apusidse 

 and Estheridae, or may become closed and partially obliterated, as in 

 the Cladocera. The right and left retinas either remain separated, as in 

 the Apusidee, or become closely approximated, as in the Estheridae, 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 Apusidse, in which the retinas are sepa- 

 rate and the optic pocket permanently open, depai't only slightly from 

 the primitive condition. In the Estheridae, in which the two retinas 

 are closely approximated, the eye is farther removed from the original 

 type ; but not so far as in the Cladocei*a, 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 hj'podermis. The three 

 groups — the Apusidse, the Estheridae, 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, tlie eyes pass 

 through stages which reproduce the essential features of the perma- 

 nent condition in the Ap\isidse and Estheridae. 



In the THIRD RETINAL TYPE, as in the more differentiated form of 

 the second, the retina is completely separated from the hypodermis. 



