MUSEUM OF COMPARATIVE ZOOLOGY. 55 
to its principal axis; distally, they terminate in the region of its base ; 
proximally, they end either at its apex or on its lateral walls near the 
apex. The 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 are covered ex- 
ternally by a second cone-shaped membrane, which separates them from 
the surrounding tissue. This membrane occupies the position of the cap- 
sular 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 differentia- 
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. The significance of these peculiarities will be discussed in the 
following paragraphs. 
The separation of the retina from the hypodermis is characteristic of 
only the more mattire conditions of the eye in Amphipods ; for as Pereyas- 
lawzewa (’88, p. 202) has shown in Gammarus, and Rossiiskaya (’89, 
p- 577, and 790, p. 89) has demonstrated in Orchestia and Sunamphitoé, 
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. 120), the distal portion of the retinal 
thickening contains almost nothing but developing cones. In sections of 
my 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 tothem. 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 
