MUSEUM OF COMPARATIVE ZOOLOGY. 59 
scribed a pair of thickenings, which extend from the superficial ectoderm 
of the antero-lateral part of the head to the brain. These thickenings 
are present only in the early stages of development, and represent the 
unsevered connection between the brain and the superficial ectoderm. 
They closely resemble the developing lateral eyes of Branchipus, and 
Grobben has therefore very justly considered them rudiments of the 
lateral eyes. If the rudiments of the lateral eyes in Cetochilus de- 
velop from the superficial ectoderm, it is probable that the lateral eyes 
in other Copepods have a similar origin. 
To which of the three retinal types already described the eyes in 
Copepods belong is not easily decided. The absence of any indication 
of an optic pocket, either in the development of what Grobben con- 
siders the rudiments of the lateral eyes in Cetochilus, or in the fully 
formed eyes in other genera, seems to me to preclude the possibility of 
these eyes belonging to what I have described as the second type. 
The separation of the retina from the hypodermis prevents them from 
being classed with the first type, and, especially in the case of the 
Branchiura, brings them into close relation with the third type. It is 
my opinion, that, if the lateral eyes in Copepods are not representatives 
of a fourth type, essentially different from the three already described, 
they must be considered members of. the third retinal type. 
Certain species of Cumacez, Ostracods, and Cirripeds possess optic 
organs which probably represent the compound eyes of other Crusta- 
ceans ; but so far as I am aware, the relation of these structures to the 
hypodermis is unknown. It is therefore impossible to state whether 
those eyes represent other retinal types, or belong to one of the three 
already described. 
According to the preceding account, three retinal types can be dis- 
tinguished in the compound eyes of Crustaceans. In the first of these 
the retina is a simple thickening in the superficial ectoderm (hypo- 
dermis). This type is characteristic of the eyes in ‘Isopods, the Bran- 
chiopodide, the Nebaliz, Stomatopods, Schizopods, and Decapods. In 
the Isopods, the eyes are sessile ; in the other groups of the first type, 
they are borne on the distal ends of movable optic stalks. 
In the second type, although the retina, as in the first type, originates 
as a thickening in the superficial ectoderm, it ultimately becomes en- 
closed within an optic pocket. This may remain permanently open, as 
in the Apusidz and Estheride, or it may become closed, as in the 
Cladocera. In the Apusidz, so far as I am aware, the eyes are not 
