Genus Acanthomytilus

Borchsenius, 1947

Appearance in life
Scale cover of adult female elongate to very elongate, moderately convex, usually brown, with terminal exuviae. Scale cover of immature male probably similar to that of female but much smaller.

Adult female not pupillarial; body fusiform to very elongate and parallel-sided; membranous except for the pygidium; lateral lobes of free abdominal segments moderately to only very slightly developed, usually with marginal spurs present. Pygidium with two pairs of lobes evident. Median lobes well developed, quite widely separated, not zygotic; each lobe almost symmetrical, with rounded tip or notched once or twice on either side. Second lobes smaller than median lobes, sometimes lacking the outer lobule; position of third lobe sometimes marked by marginal serrations. Ventral paraphyses often arising from the second lobes and sometimes from the median lobes, slender. Gland spines on pygidium slender and elongate, occurring in pairs, a long pair always present between the median lobes; much shorter if present on prepygidial segments; sometimes replaced by conical duct tubercles, especially when situated anterior to abdominal segment II. Anus circular to transversely oval, situated in the anterior quarter of the pygidium; vulva situated near centre of pygidium. Marginal macroducts numbering 5 on either side of the pygidium; each duct enlarged, with a thick sclerotized rim and with the long axis of the orifice perpendicular to the pygidial margin; single except for a pair on segment VI; the basal-most often opened in a prominent marginal process; never present between median lobes. Dorsal macroducts smaller than marginal ducts, arranged in segmental rows. Submarginal dorsal bosses absent. Duct tubercles may be present, particularly anterior to abdominal segment II. Perivulvar pores sometimes rather few, present in 5 groups. Stigmatic disc pores present by anterior spiracles only. Antennal setae variable, usually numbering 2 or 3.