Hymedesmia (Stylopus) primitiva Lundbeck (1910) is a thinly encrusting smooth sponge, of which the live colour is unknown. Like the similar Hymedesmia (Stylopus) coriacea it lacks chelae, but is distinguished from it by the polytylote tornotes (microscopic examination). It was originally described from deep water north of the Faroes, but was subsequently recorded from the Plymouth area.
Colour: Dark yellow to brown in alcohol.
Shape, size, surface and consistency: Thinly encrusting shells. Size up to 20 mm in lateral expansion, 0.5 mm in thickness. Surface smooth, some canals may be visible. Oscules circular, not elevated. Consistency soft.
Spicules: Megascleres: Ectosomal tornotes strongylote, cylindrical, polytylote: 195-280 x 2-5 µm; acanthostyles in a large size range, but not divisible in two groups, entirely spined: 120-350 x 14-27 µm (heads). Microscleres absent.
Skeleton: Hymedesmoid, with acanthostyles erect on the substrate and bundles of tornotes reaching the surface.
Ecology: Down to 750 m.
Distribution: Plymouth, Faroes, Davis Strait, Denmark Strait.
Etymology: Lundbeck gives no indication why he named this material "primitiva".
Type specimen information: The type is in the Copenhagen Museum.
The specific distinctness of this species from the common Hymedesmia (S.) coriacea remains to be further determined, as on paper both are quite similar. The polytylote condition of the tornotes is occasionally found also in H. coriacea.
Source: Lundbeck, 1910.