This slender elongated mollusc does not have a shell and takes the form of a long tube, coiled like a spring, or tied in knots, and filled with eggs.

[2][3] It holds the record as the longest gastropod mollusc in the world, one individual having been found to be 128 cm (50 in) long when uncoiled.

[4] Thyonicola dogieli is a parasite of sea cucumbers such as Cucumaria miniata.

[3] On encountering sea water the cocoons burst open and the free-living larvae are liberated.

They are about 0.1 mm (0.004 in) long, and unless swallowed by a sea cucumber, they will soon die, However, any that are ingested undergo metamorphosis in the host's gut, lose their shell and mantle and burrow into the wall of the gut with the aid of a glandular secretion.