PA RASJTIC COPEPODS— WILSON. 



843 



in the center of its posterior border, and in the partial migration of 

 the abdomen to the ventral surface of the genital segment so that 

 only a portion of it can be seen in dorsal view (fig. 13). This migra- 

 tion is apparent and not real; what actually happens is that the 

 dorsal surface of the genital segment grows back over the abdomen, 

 so that the latter appears to have moved forward along the ventral 

 surface of the former. 



Total length, 4 mm.; length of carapace on midline, 1.66 mm.; 

 width, 2.1 mm.; length of free 

 segments, 0.75 mm.; length 

 of genital segment, 1.40 mm.; 

 width, 1.5 mm. 



(3) The third chalimus has 

 practically the adult form; 

 there has been a still further 

 shortening of the posterior 

 lobes of the carapace and a 

 corresponding widening of the 

 carapace itself (fig. 14). 



The dorsal plates now cover 

 the entire surface of the free 

 segments and overlap well 

 onto the genital segment; the 

 posterior portion of the first 

 and third pairs is distinctly 

 toothed. 



There has been a further 

 shortening of the free seg- 

 ments and a further enlarging 

 of the genital segment, while 

 the median posterior sinus of 

 the latter has narrowed and 

 deepened. 



The abdomen has been en- 

 tirely covered by the back- 

 ward growth of the genital segment, so that the only thing which 

 can be now seen in dorsal view is the tips of the anal laminte, and 

 they are seen through the posterior sinus of the genital segment. 



The reproductive organs are now well formed in the genital seg- 

 ment, especially those which have to do with the receiving and 

 storing of the sperm. It is probable, therefore, that the sexes come 

 together immediately at the close of this chalimus period, before 

 the female has become permanently fixed. 



Total length, 4.5 mm.; length of carapace on mid line, 2.05 mm.; 

 width, 2.95 mm.; length of free segments, 0.75 mm.; length of gen- 

 ital segment, 1.8 mm. ; width of same, 2.6 mm. 



Fig. 14.— a chalimus of Perissoi-us communis, 

 fully developed. 



