18 



BULLETIN 15 8, UNITED STATES NATIONAL MUSEUM 



Female. — Carapace elliptical, one-third longer than wide, lateral 

 lobes not reaching the urosome; swimming legs without flagella. 

 Urosome elongate-elliptical, cut to the center or beyond, its lobes 

 lanceolate-acuminate; caudal rami basal. Teeth on basal plate of 

 maxillipeds short and sharp; anterior portion of each respiratory- 

 area much smaller than posterior ; each supporting rod of maxillary 

 disks made up of 10 segments, the basal one elongate, narrow, and 



bent, the others transversely lunate 

 and diminishing in size distally. 

 Total length, 6-12 mm. Width of 

 carapace, 3-6 mm. 



Male. — Size, shape, and general 

 proportions of metasome same as 

 in the female; urosome relatively 

 longer and narrower, the testes 

 extending beyond the base of the 

 posterior sinus. Distal segment of 

 fourth basipod with a short almost 

 spherical peg ; semen vesicle in basi- 

 pod of third legs, no other sexual 

 modifications. Total length, 5-9 

 mm. Width of carapace, 3-5 mm. 

 ReTTiarks. — From the above dis- 

 tribution it will be seen that this species is found along our entire 

 Atlantic coast, and although the alewife is its normal host, it may 

 also be looked for upon other fishes. 



Suborder CALANOIDA 



Fifth thoracic segment firmly attached to the fourth, but forming 

 a movable articulation with the sixth, Metasome considerably 

 depressed and much wider than the urosome, which is abruptly nar- 

 rowed. Female genital openings paired on the ventral surface of the 

 genital segment, male opening single, placed asymmetrically on the 

 left side. Anterior antennae elongate, many jointed, the right or the 

 left one in the male usually transformed into a grasping organ. 

 Posterior antennae biramose, endopod usually 2-segmented, its termi- 

 nal segment 2-lobed and armed with many plumose setae. Eggs 

 carried in a single ovisac attached to the ventral surface of the geni- 

 tal segment, rarely deposited singly. Includes free-swimming forms 

 only, which mostly frequent the open ocean, floating at or near the 

 surface, often in immense shoals, but sometimes found in fresh or 

 brackish water. 



Remarks. — In this group the fifth legs are normally made up of a 

 2-segmented basipod and one or two rami, also more or less seg- 

 mented. In the key to the genera of the group (Appendix B, p. 540), 



Figure G. — Argulus alosae: a. Female, 

 dorsal ; b, supporting ribs of sucking 

 disk 



