ELEMENTARY BIOLOGY. [XII. 



fish, 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 ambu- 

 latory 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 maxilli- 

 pedes 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 maxillce. 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 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 deep median fissure into two lobes, which is the metastoma. 

 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 direc- 

 tion, and consequently looks forwards. This bend of the 

 ventral wall of the body is the cephalic flexure. In corre- 

 spondence with this change of position of the surface to 

 which they are attached, the three pairs of appendages of 

 the somites which lie in front of the mouth are directed 

 either forwards, or forwards and upwards. The posterior 



