MUSEUM OF COMPARATIVE ZOOLOGY. 55 



to its principal axis ; distally, they terminate in the region of its base ; 

 proxiinally, they end either at its apex or on its lateral walls near the 

 apex. Tlie rhabdomes lie within the cavity of the cone, i, e. they are 

 distal to the intercepting membrane, as in other Amphipods. The retin- 

 ular nuclei cover the apical portion of the external surface of the cone, 

 i. e. they are proximal to this membrane. These nuclei ai-e covered ex- 

 ternally by a second coue-shaped membrane, which separates them from 

 the surrounding tissue. This membrane occupies the position of the cap- 

 suhir membrane of other Amphipods, and is unquestionably homologous 

 with it. 



The fact that both the lateral and dorsal eyes in Phronima are con- 

 structed upon the same plan as the single eye in Gammarus, supports 

 the view that these two eyes have arisen by the division of a primitively 

 single retina into two parts, and the subsequent independent difierentia- 

 tion of each part. 



As the preceding account shows, in all Amphipods whose eyes have 

 been studied carefully, the retinas conform to one structural type well 

 exemplified by Gammarus. In this type the retina is characterized by 

 two peculiarities : first, it is not continuous with the hypodermis, but 

 lies immediately below that layer ; and secondly, it possesses what appear 

 to be two basement membranes, the capsular and the intercepting mem- 

 branes. Tlie significance of these peculiarities will be discussed in the 

 following paragraphs. 



The sepai'ation of the retina from the hypodermis is characteristic of 

 only the more mature conditions of the eye in Amphipods ; for as Pereyas- 

 lawzewa ('88, p. 202) has shown in Gammarus, and Rossiiskaya ('89, 

 p. 577, and '90, p. 89) has demonstrated in Orchestia and Sunamphitoe, 

 the retina originates as a thickening in the superficial ectoderm, in the 

 same manner as in the majority of Crustaceans. So far as I am aware, 

 however, no one has observed the detachment of the retina from the 

 hypodermis, a process which must take place before the adult condition 

 is reached. In the figure of the developing eye in Gammarus given by 

 Pereyaslawzewa ('88, Plate VI. Fig. 1 20), the distal portion of the retinal 

 thickening contains almost nothing but developing cones. In sections of 

 ray own from a corresponding region in a young specimen of Gammarus, 

 the distal portion of the retina contains not only developing cones, but 

 also isolated nuclei, which occasionally lie between the cones, but more 

 frequently occur in positions distal to them. These nuclei are as numer- 

 ous in the centre of the distal face of the retina as on its edges, and at 

 this stage can always be easily distinguished from the nuclei of the cone 



