Vill INTRODUCTION. 
the head of its normal form. In the Jsopoda generally, 
the lenses of the eyes are well developed, and lodged in 
the texture of the skeleton of the animal, which is fre- 
quently thinned out to an extreme tenuity, and marked 
with numerous facets, corresponding with the many lenses 
belonging to the organ. Inthe Amphipoda, the lenses 
either are not so numerous or are less apparent, and the 
dermal tissue that covers the organ is thick and un- 
changed in character. This condition is carried to the 
greatest limit in the Phowxides, Ampeliscides, and those 
Gammarides that are inhabitants of deep and dark wells, 
where no rudiments of eyes are apparent, except in the 
presence of some coloured and ill-defined pigment cells, 
which in the Phowxides coalesce into a single organ. In 
the genus Ampelisca this pigment of colouring is associated 
on each side with two solitary lenses, that appear to be 
built into, and form part of, the dermal covering. 
It appears to be a law in the decreasing structural 
importance of Crustacea, that the segment supporting the 
appendages shall disappear before the appendage that it 
supports. In the Sessile-eyed legion, the eyes alone 
remain, the segment and the articulating portion of the 
appendage not being developed; the eyes are developed 
in most families so deeply within the head, that they 
generally appear to be behind the antennz, and some- 
times, as in Phoxus, at the extremity of the frontal 
rostrum ; in others, as Hricthonius, on a projecting lobe 
of the head, situated between the two pairs of antenne, 
in which position, owing, probably, to the insufficient 
depth of structure, the eye is borne on the internal 
surface, where it is lodged as a protuberance. But what- 
ever may be the position of these organs, the variable- 
ness of situation can only be consistent with certain 
advantages under peculiar conditions. 
In the young animal the number of facets is fewer in 
