48 BULLETIN OF THE 
three types of retinal structure can be distinguished, depending upon 
the ultimate form which this thickening assumes. 
The First TYPE which will be described is in several particulars 
the simplest, and probably represents a primitive form from which 
the other two are derived. This type is characteristic of the eyes 
in Decapods, Schizopods, Stomatopods, Isopods, the Nebaliz, and the 
Branchiopodide, and is represented by a simple thickening in the super- 
ficial ectoderm. ’ 
 _Branchiopodide. —In the eye of adult specimens of Branchipus the 
retina is a lenticular thickening occupying the inner concavity of the 
distal end of the optic stalk. Near its edges the retina is directly con- 
tinuous with the adjoining hypodermis. Its proximal face is bounded 
by a basement membrane which is also continuous with the corre- 
sponding membrane of the hypodermis, and its distal face is closely 
applied to the inner surface of the superficial cuticula. ‘Thus the retina 
in the adult has in every respect the appearance of a simple thickening 
in the hypodermis. 
The way in which the retina originates in Branchipus confirms the 
opinion that this organ has the simple structure suggested in the fore- 
going paragraph. The development of the retina in this genus has been 
studied by Claus (’86, p. 309), whose account can be summarized as 
follows. In that part of the head from which the optic stalks eventu- 
ally arise, the ectoderm becomes considerably thickened ; this thickening 
is subsequently divided into a superficial and a deep portion ; the latter 
sinks into the head and becomes a part of the central nervous system ; 
the former retains its external position and is converted into the retina. 
In Branchipus, therefore, the retina originates as a simple ectodermic 
thickening which retains its superficial position throughout the life of 
the individual. This method of origin, and the position permanently 
retained by the retina, are the two principal characteristics of the first 
retinal type. 
Isopoda. —In adult specimens of Idotea irrorata, as sections perpen- 
dicular to the external surface of the eye show (Plate V. Fig. 49), the 
retina bears the same relation to the hypodermis as it does in Branchi- 
pus. Similar structural relations occur also in the eyes of Idotea ro- 
busta and of young specimens of Serolis Schythei. 
The development of the retina in Isopods has been observed by Dohrn 
and Bullar. As early as 1867, Dohrn (67, p. 256) described the eye in 
Asellus as originating in connection with a thickening in the lateral 
wall of the head, presumably in the ectoderm of that region. The de- 
