THE COPEPOD GENUS PARANTHESSIUS — ILLG 



403 



Figure 33. — Paranthessius columbtae (Thompson): Female, A, body outline (X 28); B, 

 antennule (X 114); C, antenna (X 240); D, maxilliped (X 707); E, terminal segment of 

 fourth endopodite (X 240); F, fifth leg (X 240). 



somite of the first swimming leg (the second thoracic somite) is sep- 

 arated from the cephalothorax and is the longest pedigerous segment. 

 The articulation with the cephalothorax is very distinct both dorsally 

 and ventrally. The legs are placed far back on the ventral side of the 

 segment, and so the space between the mouth parts and the swimming 

 legs is of considerable extent. The third thoracic segment is slightly 

 shorter than the second and is the only metasome segment exhibiting 

 posterior prolongation of its lateral margins. The fifth thoracic seg- 

 ment is the terminal segment of the metasome and is much the shortest 

 and narrowest of these segments. The urosome is slightly longer than 

 the metasome. It consists of the segment of the fifth legs, the genital 

 segment and three free abdominal segments. The ovisacs are held 

 closely appressed to the body and they extend slightly beyond the tips 

 of the caudal rami. An estimate based on a typical ovisac would indi- 

 cate as the usual contents approximately 120 small eggs. 



The rostrum is highly characteristic. It is the well-developed pre- 

 hensile organ typical of the genus, with the unique feature that the 

 anterior, baselike structure of the beak is produced laterally in wide 

 expansions, the extremes of which exhibit further ventral prolonga- 

 tion, so that the principal beak is accompanied by two auxiliary knobs 



