Elpidiidae Théel, 1882

Ogawa, Akito, Morita, Takami & Fujita, Toshihiko, 2020, Elpidia soyoae, a New Species of Deep-sea Holothurian (Echinodermata) from the Japan Trench Area, Species Diversity 25, pp. 153-162 : 156

Family Elpidiidae Théel, 1882 Genus Elpidia Théel, 1876 [New Japanese name: Kuma-namako-zoku]

Elpidia Théel, 1876: 1–7 ; Théel 1877: 1–30; Belyaev 1971:

329–330; Hansen 1975: 172–173; Rogacheva 2007: 368–

372.

Tutela Perrier, 1896: 901 .

Diagnosis [modified from Hansen (1975) and Rogacheva (2007)]. Tentacles 10, having discs with long and slender, retractile processes. Tube feet 4 pairs (5 pairs for E. decapoda ), cylindrical, prominent, well spaced and equal in size. Papillae conical, separate, present along the entire dorsal radii, sometimes divided into an anterior and posterior groups. Elpidia - type ossicles (=rods with two paired horizontal arms and one paired vertical apophyses; Gebruk 1990) present; wheels occasionally present; C-shape ossicles absent. Calcareous ring strong and continuous, each piece with four pairs of arms.

Type species. Elpidia glacialis Théel, 1876 .

Remarks. The remarkable morphological characters of genus Elpidia are four paired tube feet and mainly Elpidia - type ossicles ( Gebruk 1990) in dorsal and ventral body walls. Although E. decapoda has five paired tube feet as an exception, E. decapoda has only Elpidia - type ossicles and shares the other diagnostic characters. Elpidia- type ossicles were also founded in the genera Penilpidia Gebruk, 1988 and Protelpidia Gebruk, 1983 , but Elpidia differs from these two genera in the combination with other type ossicles ( Rogacheva 2007): Penilpidia and Protelpidia have C-shape ossicles, while they do not occur in Elpidia . Therefore, the diagnosis of genus Elpidia was modified and written clearly the exceptional number of tube feet for E. decapoda , and the absence of C-shape ossicles.