20 ELEMENTARY ZOOLOGY 



their respective legs. Place all of the appendages in order on a 

 piece of cardboard. 



Examine the abdominal appendages, called pleopods, 

 or swimming feet. How many pairs are there ? Each 

 is composed of a basal part, the protopodite, and two 

 terminal segments, an inner one, the endopodite, and an 

 outer, the exopodite. 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 abdominal 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 appen- 

 dages 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 mandibles and the others are the 

 maxillcz; 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 mandibles, 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 scaphog- 

 nathite) 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 



