COPEPODS OF THE WOODS HOLE REGION 



169 



Male. — Shorter and narrower than the female; fourth and fifth 

 segments distinctly separated ; urosome 5-segmented and nearly sym- 

 metrical, the right caudal ra- 

 mus a trifle larger than the 

 left, with a tuft of stiff bris- 

 tles on the outer margin ; end 

 segment of left fifth leg 

 twice as long as penultimate 

 segment; thumb of chela on 

 right leg club-shaped, with 

 subterminal spine. Total 

 length, 0.75-0.95 mm. 



Remarks. — This species 

 may be recognized by the 

 prevailing symmetry of the 

 body and by the details of 

 the fifth legs. Williams 

 found it abundant in Nar- 

 ragansett Bay and Charles- 

 town Pond, and further 

 search will probably reveal 

 its presence in other ponds 

 around Woods Hole beside 



. Figure 115. — Tortanus setaoaudatus : a. Female, 



the two mentioned above. dorsal ; i, female, fifth legs ; c, male, fifth legs 



Suborder HARPACTICOIDA 



Fifth thoracic segment firmly attached to the sixth segment, but 

 forming a movable articulation with the fourth segment. Posterior 

 body usually about as wide as the anterior, and both divisions more 

 or less cylindrical. Genital openings paired in both sexes, and on 

 the ventral surface of the genital segment ; one or two ovisacs. First 

 antennae short, rarely more than 8- or 9-segmented, both prehensile 

 in the male ; second antennae biramose, the exopod 1- to 7-segmented. 

 rarely obsolete; mandibles and first maxillae with a palp; second 

 maxillae with digitiform processes; maxillipeds usually prehensile. 

 First legs in the majority of cases more or less transformed into 

 grasping organs, next three pairs natatory ; fifth legs never natatory, 

 but lamellar and usually 2-segmented, the basal segment enlarged on 

 its inner margin into a broad expansion, both segments armed with 

 plumose setae or spines, or both. 



In this group, as in the Calanoida, the number of segments in the 

 rami of the swimming legs is not always constant in all species of a 

 given genus ; they also sometimes differ in the two sexes of the same 



