128 
GAMMARnm 
lenses, four in number, two upon each side of the bead, 
fixed in the integument itself. Behind these fixed eyes 
is a mass of dark -red pigment : close observation, with 
a microscope, exhibits this as connected with the organs 
of sight, threads of white pigment ramifying over the 
red mass, and appearing to consist of two sets, one be- 
longing to each organ, although only partially connected ; 
sufficiently so, however, to show that the two lenses 
belong to one and the same organ of vision. 
The number of lenses belonging to the eye of an Am- 
phipod increases with the growth of the animal ; to such 
an extent is this the case, that whereas in the larva of a 
Gammarus, we have counted but eight, we find that in 
the adult there are no less than forty in each eye. We 
therefore consider that, in this genus, an arrest has taken 
place at an early stage, and limited the number of lenses 
to two, and thus produced an apparently imperfect organ 
of vision. 
The superior antennas are longer than the peduncle of 
the inferior, and are about one-third the length of the 
animal; the first joint of the peduncle is short and 
stout, and carries several fasciculi of hairs upon the 
inferior margin ; the second is twice as long, and not so 
broad, and furnished upon the inferior margin with many 
fasciculi of hairs; the third joint is very short, and in- 
creases in diameter towards the distal extremity ; the 
flagellum is about twice the length of the peduncle ; the 
first articulus is as broad at the base as the last joint of 
the peduncle, and nearly as long ; it gradually tapers to 
the distal extremity, the under margin is thickly crowded 
with auditory cilia of peculiar form ; these are long and 
slender, the basal half broader than the distal, and the 
extremity furnished with a minute denticle ; the rest 
of the articuli are small and slender, each carrying, above 
