XX INTRODUCTION. 



whenever they exist, presupposes a segment or ring to 

 which they belong. In many cases, where a coalescence 

 takes place between any of the contiguous segments, 

 their distinct existence can only be predicated by the 

 occurrence of the members which belong to them ; thus, 

 in the Brachyura, the carapace involves not only the third 

 and fourth rings, enormously developed, but also the first 

 two, which bear the eyes and antenna;, and which are 

 indissolubly blended with the succeeding ones. 



Normally there are twenty-one pairs of appendages or 

 limbs : generally speakinir, even in the higher forms, 

 twenty only are perceived, as the terminal joint of the 

 abdomen, which forms the central piece of the fan-like 

 fin, has none which are perceptible. I have, however, 

 observed them frequently in the common prawn, Palce- 

 mon scrratm,* in the form of extremely minute points 

 attached to the very extremity of the segment, and 

 moveable. 



The first pair exist only in the Podophthalma or stalk- 

 eyed forms, and constitute the peduncles upon which the 

 eyes are elevated ; they are moveable, and in many cases 

 are of considerable length, lying, when at rest, in grooves, 

 or sockets, formed for their reception. The two following- 

 pairs are of great importance, forming, in most cases, 

 organs of sense. These are the antennae One or both 

 pairs exist in all the forms of true Crustacea; ordinarily 



s I have often separated the whole twenty-one pairs of appendages in this 

 species, and placed them seriatim on a card. They consist very clearly of the 

 ocular peduncles, the anterior and posterior antenna;, the mandibles, the two pairs 

 of maxilla-, the three pairs of foot-jaws, the five pairs of thoracic legs, the five 

 pairs of abdominal false feet, the appendages to the sixth abdominal segment 

 forming tin: lateral caudal flap, and the two minute rudimentary appendages 

 above alluded to. 



