Endoparasite of some sea-cucumbers such as Holothuria poli Delle Chiaje, 1824. The latter lives mainly on infralittoral bottoms with rocky substrate and meadows, but has been also recorded deeper, on muddy grounds of the circalittoral, down to 250m deep (DORIS). Hence, E. mirabilis may be encountered from shallow water down to the continental shelf.
 
This is the larval stage of Helicosyrinx parasitica Baur, 1864. At the adult stage, the shell disappears and the animal is nude, attached to the ventral blood vessel of its host; it can reach 8cm long. Above: the shell of a veliger, found in beach grit, Plage de la Vieille Nouvelle, north of Port-la-Nouvelle, Aude, Occitania, S. France. 1,25mm. Alain Bertrand legit (FR).

Adult mirabilis (“Ent”) inside the body of the Holothurian Oestergrenia digitata (Montagu, 1815) – intertidal to 70m deep – in C. Vaney: L’adaptation des gastropodes au parasitisme, Paris 1913, p.42. Based on a drawing by Baur, this figure shows the animal fixed by its mouth on the vessel, downstream the stomach (“st”) of the host.

The mouth (b) is at right, fixed on the vessel vv — i: intestine — ov: ovarium — ps: hatchery — vo: one ootheca — t: testicle — cl: unknown (according to Baur, this extremity is closed). In the ovarium, each ootheca contains some 20 eggs; therefore, this drawing suggests the presence of circa 700 larvae in the ovarium. As Baur found one infested Oe. digitata for 200 individuals, one can conclude that there are 7 possible veligers of mirabilis for 2 encountered digitata.