342 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



Vol. xxxiii. 



On the posterior margin of the carapace just inside the posterior 

 lobes is a small fold of skin on either side, similar to that found in 

 the male. The second thorax segment does not fill the entire space 

 between the posterior lobes of the carapace, but leaves an interval 

 on either side as in the male. 



The first dorsal plates appear as lateral lobes on the sides of this 

 segment. The third segment a little more than fills the space between 

 these lateral lobes of the second segment and overlaps them on either 



side. At this stage there is no 



plates 



on 



the 



trace of dorsal 

 third segment. 



The fourth thorax segment is 

 wider than the third and slightly 

 wider than the genital segment, 

 and the dorsal plates on it ap- 

 pear as large circular lobes at its 

 posterior corners. 



The genital segment is sub- 

 quadrangular, with slight round- 

 ed projections at the posterior 

 corners and no median posterior 

 sinus. 



The abdomen is nearly half 

 the width of the genital segment 

 and projects for its entire length 

 behind the latter; the anal 

 laminae are triangular and armed 

 with short and weak nonplumose 

 spines. The swimming legs are 

 like those of the adult in every 

 particular except size (fig. 12). 

 All the other appendages also 

 are the same save the mouth 

 tube; this is broader at the tip 

 and closely resembles the form 

 seen in the metanauplius. 



The flexible lips have not yet rolled in around the mandibles and 

 given the tube its final stiletto shape. 



Total length, 3 mm.; length of carapace on mid-line, 1.35 mm.; 

 width, 1.72 mm.; length of free segments, 0.72 mm.; length of genital 

 segment, 0.77 mm.; width of same, 0.78 mm. 



(2) The second chalimus, 4 mm. in length, differs from the first 

 chiefly in the shortening of the posterior lobes of the carapace; in the 

 development of the dorsal plates on the free thorax segments and 

 the relative shortening of the segments themselves ; in the enlarging 

 of the genital segment and the formation of a deep and wide sinus 



Fig. 13.— a c;h.4.limus of Perissopus communis, 

 one-third larger than the one in fir. 11. 



