II.] THE CRAYFISH AND LOBSTER. 175 



may be observed. There is no trace of any corresponding 

 divisions in the cephalothorax of the Lobster; but, in the 

 Crayfish, the last thoracic somite is incompletely united 

 with those which precede it. The four posterior pairs of 

 thoracic limbs are those by which the animal walks and are 

 termed the ambulatory legs. The next pair in front is formed 

 by the great claws or chelue. The anterior three pairs are 

 bent up alongside the mouth and are moved to and from 

 the median line so as to play the part of jaws, whence they 

 are termed foot-jaws or maxillipedes. The external or third 

 pair of these maxillipedes are much stouter and more like 

 the ambulatory limbs than the rest, and the inner edges of 

 their principal joints are toothed. The innermost or first 

 pair of maxillipedes are broad, foliaceous and soft. When 

 these foot-jaws are taken away, two pairs of soft /bliaceous 

 appendages come into view. They are attached to the 

 hinder part of the cephalon and are the jaws or maxilla. 

 The second, or outermost, is produced, externally, into the 

 scaphognathite, which will be seen to lie in a groove sepa- 

 rating the head from the thorax laterally and known as 

 the cenncal groove. 



Anterior to these maxillae lie the two very stout mandibles. 

 Between their inner toothed ends is the wide aperture of 

 the mouth, bounded, in front, by a soft shield-shaped plate, 

 the labrum ; and behind, by another soft plate, divided by a 

 median fissure into two lobes which simulate appendages 

 it is termed the metastoma. Thus far, the surfaces of the 

 somites to which the appendages are attached look down- 

 wards, when the body is straightened out and the carapace 

 is directed upwards. But, in front of the mouth, the wall of 

 the body to which the appendages are attached is bent up, 

 at right angles to its former direction, and consequently 

 looks forwards. This bend of the ventral wall of the body 



