NO. 1788. 



NORTH AMERICAN ERGASILIDJE- WILSON. 



273 



The abdomen is always narrower and 

 nearly always longer than the genital 

 segment, and is three-jointed in the 

 great majority of species. In the gen- 

 era Irodes and PJiagus, however, it is 

 four-jointed, while in the genus Tucca 

 it has but a single joint. 



In adult females segmentation is 

 usually indistinct, being indicated only 

 by notches along the lateral margins 

 without a continuous groove. In the 

 males and in the females of a few 

 species the grooving is more distinct. 



The anal laminse are long and nar- 

 row, and the setae with which they are 

 armed are usually much longer than 

 the entire abdomen. We have thus 

 in the general body form a cephalon 

 bearing six pairs of appendages, a 

 thorax of six segments, each bearing a 

 pair of swimming legs, the sixth pair 

 often lacking in the adult female, and 

 an abdomen of one, three, or four seg- 

 ments, the last of which bears the 

 anal laminae. 



The appendages. — There are 12 pairs 

 of appendages, namely, two pairs of 

 antennae, one pair of mandibles, two 

 pairs of maxillae, one pair of maxilli- 

 peds, and six pairs of swimming legs. 

 These are all on the ventral surface 

 with the exception of the first an- 

 tennae, which arise from the frontal 

 margin and curve upward toward the 

 dorsal surface. 



There is no one genus, however, in 

 which all these appendages are pres- 

 ent and normally developed. One or 

 more pairs are often lacking, such as 

 the maxillipeds, or the sixth pair of 



swimming legs. Again, the first max- fig. 4.-ventral surface of t^niacan 

 illae and the fifth pair of legs, which 

 are always present, are also always 

 more or less rudimentary (fig. 4). 



The antennules or first antennae are 

 attached to the frontal margin, a little 

 Proc.N.M. vol.39— 10 20 



THUS CARCHARI-E, SHOWING APPENDAGES 

 (AFTER SUMPF). an', FIRST ANTENNA; 



an", SECOND antenna; g s. genital seg- 

 ment; md, mandible; mx. h., maxillary 

 hook; mx\ first maxilla; mi", second 

 maxilla; mxp, maxilliped; 1to5, swim- 

 ming legs. 



