Branch Arthropoda 55 



division comprises the remaining part of the animal, 

 and is called the abdomen. Fix carefully in mind the 

 precise boundary of these two great divisions, by bend- 

 ing the abdomen back and forth to see its mode of union 

 with the cephalothorax. The broad, unsegmented cov- 

 ering of the cephalothorax is the carapace, and its 

 toothed projection between the eyes is the rostrum. 

 Near the middle of the carapace find a distinct trans- 

 verse suture ; this represents a somewhat indistinct divi- 

 sion between the head and thorax, and for that reason 

 is called the cervical suture. 



III.--— Abdominal Somites. Find the seven distinct ring- 

 hke segments, or somites, that form the abdomen. In 

 any single somite distinguish the following parts : 

 (i) The broad, dorsal part is the tergite. (2) The nar- 

 row, ventral part of the ring is the sternite. (3) The 

 pointed, somewhat curved part which forms each side 

 is the pleurite. Notice how the seventh, or the last, 

 segment, the telson, differs from the others. 



IV. — Abdominal Appendages. On the sternite of each 

 segment find a pair of jointed movable organs, the 

 swimmerets. Examine a single one of these, and find 

 that it is made up of a basal part, the protopodite and 

 two branches ; the outer branch is the exopodite and 

 the inner branch is the endopodite. Examine other 

 swimmerets to confirm this structure. Those of the 

 first and second abdominal segments are usually modi- 

 fied, and in the fgmale are sometimes wanting. The 

 swimmerets of the sixth segment are so much enlarged, 

 and so like the hard parts of the skeleton, that they 

 will not at first be recognized as swimmerets. The 

 exopodites and endopodites of these swimmerets to- 

 gether with the telson form the tail fin of the lobster. 



