148 

 Gen. 29. Acartia, Dana, 1846. 



Syn: Diets, Lilljeborg. 



Generic Characters. Form of body slender and elegant. Cephalosome 

 attenuated anteriorly, with the front unarmed, or in some cases carrying 2 deli- 

 cate tentacular filaments below. First segment of metasome much larger than the 

 others; lateral parts of last segment generally rounded. Urosome of moderate 

 size, genital segment in female comparatively large, penultimate segment in male 

 very short, and imperfectly defined from the last one. Caudal rami of different 

 form in the different species, and, as a rule, shorter in male than in female; 

 appendicular bristle well developed and finely plumose, arising with a bulbous 

 base from the dorsal face of the rami. Eye comparatively large and placed close 

 to the front. Anterior antennae in female consisting of 17 or 18 articulations, 

 some of the bristles rather elongated and partly plumous ; those in male with 

 some of the articulations confluent, middle section of right antenna very slightly 

 tumefied, terminal section consisting of 3 articulations. Posterior antennae with 

 the junction of the inner ramus with the basal part imperfectly defined, and having 

 the distal joint unusually prolonged, outer ramus consisting of 3 joints only, the 

 outer 2 very small. Anterior lip trilobate, with the middle lobe very prominent. 

 Mandibles not very strong, outermost cutting tooth larger than the others and 

 claw-shaped, palp with the inner ramus imperfectly separated from the basal part 

 and, like the outer, carrying very long and slender seta 1 . Maxilla? with a single 

 appendicular lobe, inner ramus of palp replaced by a plumose seta, outer ramus 

 large and reflexed, with very long setse. Anterior maxillipeds short and stout, 

 with long curved spines anteriorly, digitiform lobes well developed. Posterior 

 maxillipeds resembling those in the Pontellitltr. Natatory legs without any plumose 

 seta inside the 1st basal joint, 2nd basal joint of 4th pair carrying a slender 

 deflexed seta at the outer corner; outer ramus in this and the 2 preceding pairs 

 without distinctly defined spines outside, each of the joints being only produced 

 at the end to a short dentiform projection, apical spine very slender, sword- 

 shaped, with the outer edge closely serrate. Last pair of legs in female 3-arti- 

 culate, 2nd joint somewhat dilated and carrying a long plumose seta outside, 

 terminal joint gradually exserted to a slender point, which in some cases is spini- 

 form, in others setiform; those in male 4-articulate and somewhat asymme- 

 trical, right leg the larger, with some of the joints lamellarly expanded inside, 

 terminal one securiform or slightly hooked; left leg with the terminal joint some- 

 what spoon-shaped. 



