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 Nebalise, and the 

 Branchiopodidse, and is represented by a simple thickening in the super- 

 ficial ectoderm. 



Branchiopodidce. — 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 in-orata, 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 Scrolls 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- 



