(Figure 7)
Material. Holotype: CANADA: British Columbia. Hecate Strait, Gwaii Haanas National Marine Park Reserve, 53 ˚ 32 ’ 48 ”N, 131 ˚ 19 ’ 30 ”W, 26. ix.1985, 26 meters, coll. R.O. Brinckhurst, hydrocaulus section, without gonothecae, ROMIZ B 2341. Paratype: CANADA: British Columbia. 54 ˚ 13 ’00”N, 132 ˚09’00”W, 29.iv. 1961, 98.8 meters, coll. D.B. Quayle, hydrocauli sections, without gonothecae, ROMIZ B 4070.
Reported distribution. CANADA: British Columbia, Vancouver island, off Clayoquot Sound as Sertularella rugosa.
Description. Hydrocaulus flexuous, stem geniculate; axis divided into internodes. Perisarc thin throughout. If present, ramifications arise from axis directly under axillary hydrotheca on same plane or angled perpendicularly. Ramification structure same as axis. Beginning of internode marked with two or three well-marked, oblique annular constrictions of perisarc. Internode segments narrowed basally, widening distally. Hydrothecae alternate, distant, one per internode on upper part, on same plane or slightly inclined, following ramification orientation; shape distinctly saccate basally, narrow distally. Saccate shape asymmetrical, adcauline side prominently protuberant. Adcauline wall approximately ¼ adnate, basal part of free portion fairly straight or moderately concavo-convex, distally inclined towards adcauline side. Distal portion of adcauline wall also slightly inclined outwards, forming slight concavity. Hydrothecae strongly annulated basally; apex of 3–4 annulations forming ribs or uneven, crested folds, widening towards adcauline side. Distal part of hydrothecae smooth, giving appearance of cylindrical neck, widening basally. Opercular opening rectangular, embayments shallow but flared, meeting to form four prominent cusps. Operculum of four triangular flaps, forming a low peak when closed. Intrathecal cusps not present. Gonothecae not found.
Differential diagnosis. Due to the occasionally acute angle of the outward bend, the square-shaped hydrothecal aperture, and the general rugosity of the perisarc, the hydrothecae of Sertularella sacciformis can resemble those of S. rugosa (Linnaeus, 1758) or S. tenella. ROMIZ B 2341 was originally identified as S. rugosa, but S. rugosa lacks the smooth hydrothecal neck found in S. sacciformis. The absence or presence of a notch below the rim on the abcauline wall of the hydrotheca as seen from the side is considered to be a diagnostic feature separating S. rugosa from its congeners, although its systematic value has been questioned by some authors (Cornelius 1979, 1995; Schuchert 2001). Nevertheless, in S. sacciformis this notch is absent, although the appearance of a notch is occasionally approximated by the abcauline incline of the hydrothecal neck and the tumidity of the basal portion. This gives appearance of a notch, but the area below the hydrothecal rim is actually straight. In S. tenella, the hydrotheca resembles a broad-necked bottle, rather than the saccate, bent-neck shape of S. sacciformis. The nature of the rugosity is also different in all three species. In S. rugosa the encircling transverse ridges are sharp, forming deep furrows, which are especially marked on the abcauline side; in S. tenella, the sharp ridges which encircle the hydrotheca appear more evenly spaced, but in S. sacciformis, the annular segments form uneven, crested folds, widening towards the adcauline side. Moreover, in S. rugosa the colony tends to be poorly ramified on a single plane (Naumov 1969). In S. sacciformis, the stem is slender, and the hydrothecae are not as closely spaced as in S. rugosa.
Etymology. The species name refers to the shape of the hydrothecae, from the Latin saccus (sack or pouchshaped).