128 GAiMMAlimffl. 



lenses, four in number, two upon each side of the head, 

 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 antennae 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 



