Sewelloya plana n. sp. (Figs. 14-16)

Material examined. 2 from the terebellid polychaete Loimia medusa (Savigny) collected from the intertidal zone of Choonjangdae (36 09′40′′N, 126 30′ 57′′E), in the Yellow Sea, 03 January 2014. Holotype (, NIBRIV0000286636) and allotype ( 6637), both intact, have been deposited in the National Institute of Biological Resources (NIBR), Incheon, Korea. Dissected paratype () is retained in the collection of the author .

Other material examined. 1 (both dissected) from the same host and locality as type material, 01 February 2002 .

Female. Body (Fig. 14A) dorsoventrally flat, with laterally inflated prosome and cylindrical urosome. Body length 1.19 mm in dissected paratype (1.12 mm in holotype). Prosome 775 Μm long and consisting of cephalothorax and second to fourth pedigerous somites. Greatest width of prosome 555 Μm. Cephalothorax divided dorsally by faint suture line into cephalosome and first pedigerous somite, with broad apical prominence and rounded posterolateral corners. Fourth pedigerous somite small and much narrower than preceding somite. Urosome (Fig. 14B) 5-segmented. Fifth pedigerous somite short but nearly as broad as genital double-somite, with 3 unequal setae (representing leg 5) on both sides. Genital double-somite consisting of expanded anterior part (67 Μm) and narrower distal part (50 103 Μm); these anterior and posterior parts divided dorsally by suture but not divided ventrally (Fig. 14 B-D); genital aperture locating laterally (Fig. 14C). Genital double-somite and first and second free abdominal somites with membranous flange along posterodorsal margin (Fig. 14B) and spinules along posteroventral margin (Fig. 14D). Three free abdominal somites 58 97, 58 85, and 58 67 Μm, respectively. Caudal rami slightly divergent; each ramus 92 26 Μm (ratio 3.54: 1), with 6 naked setae; seta I absent; seta II locating dorsally at halfway of ramus; seta V the largest, 396 Μm long, more than 3 times as long as next longest seta. Egg sac unknown.

Rostral area broad but rostrum absent (Fig. 15A). Antennule (Fig. 14E) 300 Μm long and 6-segmented; first to third segments gradually narrowing distally; armature formula 2, 6, 7 (one being minute), 4+aesthetasc, 2+aesthetasc, and 7+aesthetasc; all setae slender and some of them weakly pinnate. Antenna (Fig. 14F) 4-segmented; coxobasis gradually broadened distally, with 1 small medio-distal seta and minute spinules in distal region; first endopodal segment the longest, gradually broadened distally, with 1 small seta on medial margin slightly distal to midlength of the margin and 1 transverse row of minute spinules near outer distal corner; second endopodal segment small, with short claw and 2 setae, proximal one of latters minute; third endopodal segment twice as long as wide, with 3 broad claws and 2 setae distally and 2 minute setae subdistally on outer side; medial one of 2 distal setae truncate and tipped by 1 spinule.

Labrum (Fig. 15B) small, with prominent distal lobes articulated at base. Mandible (Fig. 15C) short, strongly tapering, broad at base; convex outer side with 1 claw-like spine and several minute spinules; convex side of blade with row of fine teeth; concave side of blade with curved row of fine spinules; distal end of blade rather blunt, not flexible. Paragnath not seen. Maxillule (Fig. 15D) lobate, with 4 naked setae on oblique distal margin. Maxilla (Fig. 15E) 2-segmented; proximal segment with long membranous flange on outer (ventral) side, 1 tuft of setules on proximal region and 1 curved row of fine spinules on distal region; distal segment strongly recurved in distal region bearing 5 or 6 (usually 6) strong teeth along convex outer side and 1 row of minute spinules on anterior side, with 2 setae (setae I and II); seta I (medial seta) large, nearly as long as segment and spinulose along distal margin; seta II (anterior seta) stiff and spinulose along medial margin. Maxilliped (Fig. 15F) 3-segmented; first segment (syncoxa) unarmed but with 3 transverse rows of fine spinules on medial side; middle segment (basis) with 2 spiniform setae: small, spinulose medial one and larger, unilaterally spinulose one terminating in thin, setiform tip; distal segment (endopod) small, tapering, with 2 minute setae near middle and row of fine spinules distally.

Legs 1-3 (Figs. 15G, 16A, B) biramous, with 3-segmented exopod and 2-segmented endopod. Leg 4 (Fig. 16C) uniramous, consisting of unarmed coxa, 1 setabearing basis, and 1-segmented exopod; intercoxal sclerite with pair of pointed distal processes; 7 setae on exopod stiff and spiniform. Coxa of legs 1-3 with small, pinnate inner seta and spinules on outer distal corner. Basis of leg 1 with 1 medio-distal, membranous flange near base of endopod and 2 rows of minute spinules on ventral surface. Basis of legs 2 and 3 with 2 ventral rows of minute spinules, each row near bases of exopod and endopod. Endopods of legs 1-3 with transparent knob on outer distal corner of proximal segment and outer margin of distal segment. First exopodal segment of legs 1-3 with fine spinules on outer margin. Armature formula of legs 1-4 as follows:

Leg 5 represented by 3 unequal setae (Fig. 14C); lengths of these setae 112, 38, and 22 Μm, respectively, from dorsal to medial; minute spinules present near base of setae. Leg 6 represented by 1 small, simple seta on genital operculum (Fig. 14C, D).

Male. Body (Fig. 16D) narrower than that of female. Body length 1.06 mm. Greatest width 450 Μm measuring across cephalothorax. Urosome 6-segmented. Fifth pedigerous somite slightly narrower than genital somite. Genital somite 133 Μm, with convex lateral margins, distinct genital operculum tipped by 1 seta, and deep posteroventral incision (Fig. 16E). Four abdominal somites gradually narrowing from proximal to distal. Caudal ramus 60 23 Μm (ratio 2.61: 1), shorter than that of female.

Rostrum absent, as in female. Antennule with 1 additional aesthetasc on second segment at opposite side of dot in Fig. 14E. Antenna as in female.

Labrum, mandible, maxillule, and maxilla as in female. Maxilliped (Fig. 16F) 4-segmented; first segment with 2 tapering processes at medio-distal corner; second segment with convex outer margin and 1 medial, longitudinal row of fine spinules and armed with 2 massive, transformed spines on medial margin; latter 2 spines (proximal one larger than distal one) distally expanded and spinulose, and tipped by 1 setule; third segment small and unarmed; terminal segment forming long, strongly curved claw bearing 1 large and 1 minute setae proximally.

Legs 1-5 not different from those of female. Leg 6 represented by 1 seta on genital operculum (Fig. 16E).

Etymology. The name of the new species refers to the flat body and is derived from the Latin planus (=flat).