Selenkiella siamense Heding & Panning, 1954

(Figures 5 – 7)

Material examined. Holotype: Ko Kradat, Thailand, 8–10 m, 27.i. 1900, 45 mm length (ZMUC –HOL– 250). Paratype: Koh Kram, Thailand 25.ii. 1900, 50 mm length (ZMUC –HOL–251).

Redescription. Body barrel-shaped; tube feet scattered throughout body (Fig. 5a). Color, in ethanol, gray. 25 dendritic tentacles arranged in 2 circles (15 + 10): 15 in outer circle, 10 in inner circle. Anal papillae present. Longitudinal and circular muscles well developed (Fig. 5b). Longitudinal muscle split at anterior end (Fig. 5b). Retractor muscle short, attached to middle of radial plate.

Calcareous ring robust. Radial plates slightly taller, much larger than interradial ones. Interradial plate articulated with base of radial plate and first piece of posterior process, such that half of interradial plate is connected to the posterior process (Figs. 5c; 6a). MidIR (IR5) and midR (RI) not modified (Fig. 6b, c, respectively). Radial plates undivided, rectangular, base convex, without projection, two AP, one wider than the other (Fig. 6 a; c). Posterior processes longer than radial plates, subdivided into about 3 – 4 large pieces (Figure 5c). Interradial plates elongated, with straight (Fig.6) or slightly concave base (Fig. 6b) and pointed anterior end. Marginal grooves for AB deep (Fig. 6e). TCT rectangular (Fig. 6f).

Body wall with four–pillared tables with circular disc (70–100 μm long; Fig.7 a,b); disc with one large central hole, eight marginal perforations, margin undulated; spire ending in 8–12 spines. Introvert with rosettes (30–80 μm long; Fig.7c, d). Tentacles with rods branched or perforated at both ends (former 50–100 μm long;latter 40–100 μm long). Tube feet with flat perforated plates, with irregular margins (140–200 μm long; Fig.7e) and endplate with minute holes, larger around margin and smaller medially (endplates up to 300 μm diameter; Fig.7f).

Remarks. The following illustrations are additions to the data provided by Heding & Panning (1954): the aquapharyngeal bulb attachment; longitudinal muscle attachment and retractor muscle attachment (fig 6d), the grooves in the radial plate for water vascular canals (fig 6e) and transverse cross-sections at the top region (fig 6f). We also provide a photograph of the general aspect of the body (Fig. 5a) and described for the first time by means optical microscopy the morphology of the longitudinal retractor and circular muscles Fig. 5b.