Holotype: NSMT-Ec 100, mature male, TL 25 mm, PL >5 mm (proboscis damaged), Hachi-no-higata sandy flat at the mouth of the Kamogawa River, Hiroshima Pref., Japan (34°19.42'N, 132°53.88'E), May 18, 2011, collected by M. Tanaka and T. Nishikawa. Paratypes: NSMT-Ec 101–105, 3 males + 2 females, TL 28–40 mm, PL 6 mm, collection data as for the holotype. Non-type specimens: TUM Echiurida 2-11, labeled as “Thalassema owstoni Ikeda” probably by H. Sato, 8 males + 6 females + 1 specimen of unknown sex, TL 29–73 mm, proboscis absent, Onomichi Bay, Hiroshima Pref., Japan (34°24'N, 133°12'E), March 7, 1931, collected by Takashi Gamo; TUM Echiurida 1-5, labeled as “Thalassema owstoni Ikeda” and “kouji” probably by H. Sato, 2 males + 3 females + 1 of unknown sex, TL 31–62 mm, PL 16–33 mm, Onomichi Bay, Hiroshima Pref., Japan (34°24'N, 133°12'E), March 7, 1931, by Takashi Gamo; NSMT-Ec 106, 1 of unknown sex, TL 28 mm, proboscis absent, intertidal sandy flat at the mouth of the Kamogawa River, Hiroshima Pref., Japan (34°19.42'N, 132°53.88'E), July 8, 2006, by Masanori Sato; NSMT-Ec 107, 1 of unknown sex, TL 33 mm, proboscis absent, intertidal sandy to muddy flat on Ikarise Islet, located in a channel of Lake Hamana, Shizuoka Pref., Japan (34°41.08'N, 137°35.98'E), April 19, 2003, by Shoichi Kimura; UMUTZ-Ecur-2, 1 of unknown sex, TL 55 mm, proboscis absent, above low-water mark, Tomono-ura, Hiroshima Pref., Japan (34°22'N, 133°23'E), July 1882, by I. Ikeda, reported as Arhynchite arhynchite (Ikeda) by Nishikawa (2001); UMUTZ-Ecur-10, 1 of unknown sex, TL 38 mm, proboscis absent, Misaki, Sagami Bay, Kanagawa Pref., Japan (35°09’N, 139°36’E), collection data unknown, reported as Arhynchite arhynchite by Nishikawa (2001). Specimens for comparison: Arhynchite arhynchite; LTTU-Y009, 1 female + 3 of unknown sex, TL 42–74 mm, off Abashiri, Hokkaido, Japan, 7–12 m depth, September 17, 2001, collected by Yasuhiro Kuwahara.

Trunk up to 80 mm long in preserved specimens. Leaf-like gonostomal lips with smooth margins. Neurointestinal vessel unbifurcated. Ring vessel absent. Rectal caecum absent. Anal vesicles fastened basally to the trunk wall by mesenteries.

In life, trunk colored pinkish yellow and proboscis pale yellow (Fig. 1). Coloration fading to pale white or beige after fixation with formalin.

In preserved specimens, TL ranging from 25 to 73 mm (n = 28) and PL ranging from 6 to 33 mm (n = 8). Proboscis, often detached from trunk, elongated and slightly expanded at its extremity, with its proximal end forming small lower cup around mouth. Trunk wall thickened and covered with numerous distinct papillae, especially prominent (up to 1 mm) at both extremities. Trunk musculature consisting of outermost circular, middle longitudinal, and innermost oblique layers, all of which continuous throughout.

Paired ventral setae of usual form, with strong interbasal muscle (Fig. 2A), but only very rarely single seta without interbasal muscle in 2 of 6 non-type specimens in TUM Echiurida 1-5.

Paired gonoducts situated slightly behind ventral setae (Fig. 2A). Gonostomes situated proximally, each with leaf-like gonostomal lip with smooth margin (Fig. 2B); lip plicated marginally, probably due to fixation in many non-type specimens (TUM Echiurida 1-5, 2-11).

Long and convoluted alimentary canal, filled with sand grains and elliptical fecal pellets, ca 2 mm in long axis (Fig. 2A, C). Anterior part of alimentary canal, so-called foregut, divided into pharynx, esophagus, gizzard, and crop (Fig. 2A). Intestine following foregut fastened to trunk wall with numerous thread-like mesenteries and divided into pre-siphonal, siphonal, and post-siphonal parts (Fig. 2A, C). Pre- and post-siphonal parts having a ciliated groove; elongated pre-siphonal part about twice as long as TL (Fig. 2A). Rectal caecum absent (Fig. 2C).

Vascular system composed of dorsal, neurointestinal, and ventral vessels, without ring vessel (Fig. 2A). Dorsal vessel attached to entire length of crop (Fig. 2A). Ventral vessel running along almost entire length of ventral nerve cord and terminating at posterior end of post-siphonal intestine near anus (Fig. 2A, C). Neurointestinal vessel, although injured in holotype, issuing from ventral vessel slightly behind gonostomal level and terminating at anterior end of intestine without bifurcation (Fig. 2A). In 6 non-type specimens (5 in TUM Echiurida 2-11 and 1 in TUM Echiurida 1-5), ventral vessel issuing additional branch forward at ventral origin of neurointestinal vessel, with the branch terminally forming small loop around interbasal muscle. Further, in another non-type specimen of TUM Echiurida 1-5, origin of neurointestinal vessel shifted far forward, crossing interbasal muscle but without additional branches.

Paired simple anal vesicles, ca one-third of TL in holotype, while rarely attaining TL in other specimens, covered wholly with numerous microscopic ciliated funnels and fastened basally to trunk wall by some mesenteries (Fig. 2C).

The specific name is dedicated to the late Dr Hayao Sato who made a significant contribution to the taxonomy of echiurans, sipunculans, and priapulids in Japan and adjacent waters.

Hiroshima Pref. (Seto Inland Sea), Ikarise Islet at the entrance of Lake Hamana, and Misaki (Sagami Bay), Japan, intertidal to subtidal, sandy to muddy bottoms.

Table 1 gives a comparison of all known species and the newly described species assigned to the genus Arhynchite. Arhynchite hayaoi sp. n. is distinguishable from Arhynchite arhynchite, the only congener recorded to date from Japanese coasts, by the absence of such digitiform projections or irregular serrations along the margin of the gonostomal lips as are present in Arhynchite californicus and Arhynchite pugettensis, respectively (Fisher 1949, pls. 30, 32), and by the complete lack of bifurcation in the neurointestinal vessel.

Nishikawa (2001) recorded Arhynchite arhynchite from Sagami Bay and the Seto Inland Sea. Our detailed re-examination of these specimens revealed that they are assignable to Arhynchite hayaoi, instead of Arhynchite arhynchite. Thus, it is possible that Arhynchite hayaoi is widely distributed in the Seto Inland Sea and the Pacific coasts of Middle Japan, whereas Arhynchite arhynchite, with its type locality probably originating near Hokkaido (Ikeda 1924, Satô 1937), is restricted to cold northern waters.