COPEPODS OF THE WOODS HOLE REGION 117 



Male. — Urosome usually distorted to the right, but symmetrical 

 in young males and sometimes in the adults; basal abdominal seg- 

 ment longer than the genital segment, twice as wide as long, with 

 a large winglike process extending to the right, denticulate along its 

 posterior margin, and hooked at its tip ; third segment with a shorter 

 process, and fifth segment with two processes, all on the right side; 

 caudal rami asymmetrical, the right one the longer, three times as 

 long as wide. Right fifth leg uniramose, its blunt apical claw folded 

 back along the inside of the leg ; left fifth leg biramose, the endopod 

 a long stout claw on the inner margin of the second basipod seg- 

 ment, the exopod 2-segmented. Total length, 1.5-1.75 mm. 



Development stages. — In young females the basal segments of the 

 fifth legs are indistinguishably fused with each other and with the 

 ventral surface of the fifth segment. Each leg has two free seg- 

 ments, short, stout, and armed with minute spines, the distal segment 

 twice the length of the basal. In the youngest males the basal 

 segments of the fifth legs are separated by a distinct median sinus, 

 the right leg is 3-segmented, the second segment two-fifths longer 

 than the third, both armed with short, stout spines ; the left leg shows 

 beyond question that it is biramose, the 1-segmented endopod and 

 the 2-segmented exopod attached side by side to the end of the second 

 basipod segment. In older males the right fifth leg becomes dis- 

 tinctly 4-segmented, indicating that the terminal portion in the 

 adult is really two segments fused. In the left fifth leg the endopod 

 is drawn away from the exopod, and the latter is considerably 

 lengthened, but still 2-segmented as in the fully developed adult. 



Remarks. — This copepod can be recognized most easily by the 

 three terminal setae on each caudal ramus. This character is es- 

 pecially useful in separating young females from the adults of the 

 various species of DiaptomMS with which they are nearly always 

 associated. 



Family METRIDIIDAE 



Genus METRIDIA Boeck, 1864 



Body slender and elongated; head separated from the first seg- 

 ment; fourth and fifth segments fused, with rounded or angular 

 corners but no processes or spines; urosome narrow and elongated, 

 genital segment not protuberant ventrally ; caudal rami truncated at 

 the tip, the two middle apical setae enlarged and jointed near the 

 base ; rami of the first four pairs of legs 3-segmented, basal segment 

 of second endopod deeply invaginated on the inner margin, with 

 hamif orm spines on the distal margin of the invagination ; fifth legs 

 uniramose in both sexes, 3- or 4-segmented in the female, 5-segmented 

 in the male. 



