200 



Seventeen specimens of this species were obtained from the plankton collected at the 

 following stations. 



Stat. 16, 3 specimens. — Stat. 35, 4 specimens. ■ — Stat. 217, 3 specimens. — Stat. 271, 

 4 specimens. — Stat. 304, 3 specimens. 



Microsetella rosca is readily distinguished from Microsetella atlantica by its larger size, 

 by the very long furcal setae, and by the form of the fifth pair of feet. 



This Microsetella has a moderately wide distribution in all the great oceans. 



26. Family Harpacticidae. 

 Genus Harpacticus M. Edwards, 1S38. 



The feature that separates this genus from the other members of the family is the 

 peculiar transformation of the exopodite of the third pair of feet of the male. The exopodite 

 of this pair forms a powerful prehensile appendage. 



Three species were obtained from the plankton collected by the 'Siboga' in the Malay 

 Archipelago. Two of the species do not appear to have previously been described. 



1. Harpacticus cristatus nov. sp. Plate LXI, figs. 1 — 8. 



Female — length .49 mm. 



Seen from above, the bod)- appears rather slender and depressed. It resembles a Laophoutc 

 in general appearance. The cephalic segment is large and is provided vvith a distinct crest 

 traversing the whole of the middle line. The crest is quite obvious both in the lateral and 

 dorsal positions (Plate LXI, figs. i & 2). The anterior portion of the ventral margin is considerably 

 expanded as shewn in the illustration. The rostrum is moderately wide. The apex is bluntly 

 rounded. The ventral margins of the thoracic segments are distinctly pointed at the distal ends. 



The abdomen is composed of four segments. The combined length of the abdominal 

 segments and furcal joints is contained fully twice in the total length of the ccphalothorax, 

 from the apex of the rostrum to the end of the last thoracic segment. The genital segment 

 is moderately large. It is equal to the combined length of the next two segments- The ventral 

 surface is traversed by a distinct suture. The second and third segments are of nearly equal 

 length, but the lateral margins of the third segment appear rather hollowed out. The furcal 

 joints are longer than broad (Plate LXI, fig. 8). 



The antennules are composed of eight joints. The first four joints are long and narrow. 

 The last four joints are very short The combined length of the last four joints is only about 

 one-seventh of the combined length of the first four joints. 



The antennae, mandibles, maxillae, and first pair of maxillipedes resemble those of 

 Harpacticus chelifer (Muller). The second joint of the second pair of maxillipedes is long and very 

 slender. The terminal claw is also long and slender. It is much longer than the second joint. 



The first pair of feet is rather slender. The exopodite is much longer than the endopodite. 

 1 hc first joint of the exopodite is short. lts length is contained two and onedialf times in the 



