XX INTRODUCTION. 



whenever they exist, presupposes a segment or ring to 

 which they belong. In many cases, ^Yhere 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 antennae, and which are 

 indissolubly blended with the succeeding ones. 



Normally there are twenty-one pairs of appendages or 

 limbs : generally speaking, 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 serratus,* 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 



* 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 antennas, the mandibles, the two pairs 

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

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

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

 above alluded to. 



