xii.] THE FRESH-WATER CRAYFISH. 129 



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 is formed 

 by the great claws or chelce. 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 foliaceous 

 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 which 

 separates the head from the thorax laterally and is the 

 cervical groove. 



Anterior to these maxillae lie the two very stout man- 

 dibles. Between their inner toothed ends is the wide aper- 

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

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

 by a deep median fissure into two lobes, which is the meta- 

 stoma. Thus far, the surfaces of the somites to which the 

 appendages are attached look downwards, 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 is the cephalic flexure. 

 In correspondence with this change of position of the sur- 

 M. 9 



