50 BULLETIN OF THE 



Carabarus, Crangon, and Palsemouetes. There is, therefore, considera- 

 ble ground for the support of CarrieWs 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 Branchipodida?, the 

 Nebalia?, 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 tyi>e 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 Apusidre, the Estheridae, 

 and the Cladocera. 



Estheridce. — 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 



