Family Amphiuridae Ljungman, 1867

Type genus

Amphiura Forbes, 1843.

Other genera

Acrocnida Gislén, 1926; Amphichondrius Nielsen, 1932; Amphicontus Hill, 1940; Amphigyptis Nielsen, 1932; Amphiodia Verrill, 1899; Amphiomya H.L. Clark, 1939; Amphioncus H.L. Clark, 1939; Amphioplus Verrill, 1899; Amphipholis Ljungman, 1866; Amphipholizona H.L. Clark, 1915; Amphistigma H.L. Clark, 1938; Dougaloplus A.M. Clark, 1970; Microphiopholis Turner, 1985; Nannophiura Mortensen, 1933; Nudamphiura Tommasi, 1965; Ophiocentrus Ljungman, 1867; Ophiocnida Lyman, 1865; Ophiodaphne Koehler, 1930; Ophionephthys Lütken, 1869; Ophiophragmus Lyman, 1865; Ophiosphaera Brock, 1888; Ophiostigma Lütken, 1856; Paracrocnida Mortensen, 1940; Paramphichondrius Guille & Wolff, 1984; Paramphiura Koehler, 1895; Triplodia Turner & Hallan, 2011.

Description

ARM. Arm shape unbranched; length 3–4× or more than 4× disc diameter; covering without granules/spines/tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6–0.9, or 0.9–1. Arm spines tapering (conical), cylindrical, or pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines between half and one, or between one and two segments long. Tentacle scales present or absent (Ophiophragmus); pores only proximally visible along the arm; nearly as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) only lateral. Dorsal and/or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation without dorsal and ventral lobes, or with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing.. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening.

DORSAL DISC. Disc pentagonal to circular; flat (about same level as arms) to high (above arm level). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates or with concealed primary plates; without scales; without spines or with spines, granules and tubercles in some genera (O’Hara et al. 2018). Dorsal disc scale/plate without tubercles. Radial shield present; without granules/spines; length between one third and half of the disc radius; scalene (oblique) triangular, or isosceles (mirror-symmetric) triangular to pear-shaped, or bar-like.

VENTRAL DISC. Gonads restricted to the disc. Ventral interradii without granules/spines or with spines. Scales on ventral interradii covering the whole area of interradius or absent. The number of scales on H line of an interradius about 14–21. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman’s ossicle present. Oral shield covering less than one third of interradius; much wider than long or about as long as wide (Amphipholis). Madreporite one; with one pore or with numerous pores. Adoral shield separated or meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate dorsal half widest, entire (single plate). Dental sockets slit-shaped to heart-shaped, equal to or more that 50% of the width of dental plate. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as single column.

VERTEBRAE. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.