50 BULLETIN OF THE 
Cambarus, Crangon, and Palemonetes. There is, therefore, considera- 
ble ground for the support of Carriére’s generalization, that the relation 
of the retina to the hypodermis as shown in Astacus is characteristic 
of all Decapods. 
The development of the retina has been more fully studied in Deca- 
pods, perhaps, than in any other group of Crustaceans. Nevertheless, 
the accounts given by various writers are by no means in agreement, but 
differ in several important particulars. In a former paper (Parker, ’90*, 
pp. 31-43), I devoted considerable space to the discussion of these 
accounts, and I shall therefore not reopen the subject here. Suffice 
it to say, that since the publication of the paper referred to nothing has 
transpired to alter my belief that the retina in Decapods originates as 
a simple thickening in the superficial ectoderm. 
In a recent preliminary communication by Lebedinski (90) on the 
development of a marine crab, Eriphya, a brief description of the origin 
of the eye is given. This description, however, is so very much con- 
densed that it is not easily understood, and since the author himself 
confesses that, on account of the complexity of the subject, a descrip- 
tion without figures must be almost unintelligible, it would be unwise 
to hazard a presentation of his views. I shall therefore pass over this 
paper without further comment. 
The evidence advanced in the course of the preceding paragraphs 
leaves no doubt in my mind that the retinas in the Branchipodide, the 
Nebaliz, the Isopods, Stomatopods, Schizopods, and Decapods, belong to 
the same structural type, and that this type is represented by a thick- 
ening in the external ectoderm (hypodermis), which retains permanently 
its superficial position. 
The SECOND RETINAL TYPE is more complicated than the first, and 
differs from it in that the retina does not retain its position at the 
surface of the body, but becomes buried beneath a fold of integument. 
Our knowledge of this type is largely due to the researches of Grobben 
(79). The type is represented in the eyes of the Apuside, the Estheride, 
_ and the Cladocera. . 
Estheride.—In adult specimens of Limnadia Agassizii the two lat- 
eral eyes are rather closely approximated, and occupy a position in 
the ventral anterior portion of the animal’s body (Plate IV. Fig. 33). 
The relation of the eye to the surface of the body can be seen most 
satisfactorily in sagittal sections. In such a section (Fig. 35) the eye 
has the appearance of a stalked structure which projects anteriorly into 
a cavity, the optic pocket (brs. oc.); this pocket communicates with the 
