CEPHALOMASTAX BREVIS IWATA, 1957

(FIGS 2C, 5A–E)

Cephalomastax brevis Iwata, 1957: 5–7, pl. I, fig. 9, pl. II, fig. 7, pl. III, figs 1–7; Kajihara, 2017: 423, fig. 16.2h.

Material examined: Two non-type specimens. ICHUM 6267, serial transverse sections, 8 µm thick, Mallory, 13 slides, along with extracted total DNA; 19 February 2014, dredged between 35°5′56″N, 139°34′10″E, 249 m depth, and 35°5′46″N, 139°34′04″E, 309 m depth, at station 2 of the 2 nd JAMBIO Coastal Organism Joint Survey (Nakano et al., 2015), collected by H. Kajihara. ICHUM 6304, extracted total DNA and body fragment preserved in 99% EtOH, collection data as for ICHUM 6267.

Sequences: From ICHUM 6267: LC178640, 28S (2096 bp). From ICHUM 6304: LC178651, H3 (331 bp); LC178689, 16S (510 bp); LC190961, COI (658 bp) .

Description: Two fragments of anterior body, each about 2.5 cm long and 2 mm wide. Background body colour pale peach, dorsally maroon; dorsal pigmentation not covering head (Figs 2C, 5A, B). Head not demarcated from body in normal state, but clear demarcation obvious when animal contracted (Fig. 5A, B). Transverse cephalic furrow encircling head; secondary furrows present. Proboscis pore slightly behind tip of head (Fig. 5B). Mouth behind cephalic furrow (Fig. 5B). Anterior proboscis wall composed of glandular epithelium, outer longitudinal muscle layer, neural plexus, inner longitudinal muscle layer, and circular muscle endothelium (Fig. 5C, D); longitudinal muscle fibres abutting neural plexus running diagonally, often penetrating nervous tissue (Fig. 5D). Posterior proboscis wall composed of glandular epithelium, longitudinal muscle layer and endothelium (Fig. 5C). Rhynchocoel wall composed of meshwork of interwoven longitudinal and circular (and maybe also diagonal) muscle fibres; circular component denser in inner portion (Fig. 5E).

Distribution: Known only from the type locality, Sagami Bay, Japan (Iwata, 1957; present study).

Remarks: Because Cephalomastax brevis was originally described from a preserved specimen, the external features in the living state were unknown. However, Iwata’s (1 9 5 7: 6) description of the appearance of the preserved type specimen (before sectioning) matches perfectly our observations in that the ‘body is pale chestnut in colour on the dorsal side except only the portion of the neck which is whitish; the ventral surface is white’. Other features such as the absence of eyes and the unique arrangement of the proboscis muscle layers in relation to the nerve plexus (outer longitudinal, nerve plexus, inner longitudinal, and inner circular muscle layers) add more credence to the species identification of our specimens.