ELEMENTARY BIOLOGY. [CHAP. 



The typical abdominal segment. 



Isolate, by carefully dissecting it away from its 

 fellows, the third abdominal segment and its ap- 

 pendages. Pick away its contents and examine 



a. The segment proper: arched above; flattened 

 below. It may be subdivided into 



a. a dorsal tergum, the anterior smooth portion of 

 which is overlapped by the preceding segment, 

 in extension of the abdomen. 



(3. The sternum; that portion of the ventral surface 

 which lies between the points of attachment of 

 the appendages. 



y. The epimeron: the portion of the ventral surface 

 which lies on each side immediately external to 

 the attachment of the appendage. 

 This region is very short and passes almost 

 directly into the inner wall of 



8. \h.& pleuron; a free wing-like downgrowth of each 

 lateral wall of the somite, seen, in section, to 

 be formed by both tergum and epimeron. Note 

 the smooth anterior half of the pleuron; it is 

 overlapped by the one in front during flexion of 

 the abdomen, much as is the tergum during 

 extension. 



e. The articular facets; small tubercles developed, 

 one on either side, from the anterior edge of the 

 base of the tergum, immediately above the 

 pleuron. They fit, during life, into correspond- 

 ing recesses of the posterior border of the tergum 

 in front. 



b. The appendages or swimmerets, one on each side: 

 the structure of each 



