viii INTRODUCTION. 



the head of its normal form. In the Isopoda 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. In the 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 Phoxides, 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 Phoxides 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 antennae, and some- 

 times, as in Phoxus, at the extremity of the frontal 

 rostrum ; in others, as Ericthonius, on a projecting lobe 

 of the head, situated between the two pairs of antennae, 

 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 



