THE CRAYFISH 31 



Examine the abdominal appendages, called pleopods, or 

 swimming feet. How many pairs are there ? Each is com- 

 posed of a basal part, the protopodite, and two terminal seg- 

 ments, an inner one, the endopodite, and an outer, the exo- 

 podite. In the males the first and second pleopods of the 

 abdomen are larger and less flexible than the others. In the 

 female the pleopods serve to carry the eggs and the first two 

 pairs are very small or absent. Note the last set of abdomi- 

 nal appendages. These are the uropods, which together 

 with the telson form the tail. 



Make a drawing of the pleopods of one side. 



Examine the appendages of the cephalothorax. Like the 

 appendages of the abdomen the typical composition of each 

 includes a protopodite, an exopodite and an endopodite, but 

 some of these appendages are much modified, and show a 

 loss of one of these parts, or the addition of an extra part. 

 The cephalothoracic appendages may be divided into three 

 groups, an anterior group of three pairs of mouth-parts 

 (belonging to the head) of which the first pair is the mandi- 

 bles and the others are the maxillce; a second group of three 

 pairs of foot-jaws or maxillipeds, belonging to the thorax, 

 and a third group of five pairs of walking-legs. The man- 

 dibles, lying next to the mouth-opening, are hard and jaw- 

 like and lack the exopodite; the first maxillae are small and 

 also lack the exopodite; the second maxillae have a large 

 paddle-like structure which extends back over the gills on 

 each side within the space, the branchial chamber, above the 

 gills. It is by means of this paddle-like structure (the 

 scaphognathite) that currents of water are kept up through 

 the gill-chambers. The maxillipeds increase in size from 

 first to third pair. Each pair of walking-legs except the 

 last bears gills. These gills are the organs by which the 

 blood is purified. The blood of the crayfish flows into the 

 large vessels on the outer sides of the gill and thence into 

 the fine vessels in the little leaf-like lamellae. At the same 



