Hallucigenia was found by Walcott (1911) from the Burgess Shale which is famous for yielding bizarre fauna of the Middle Cambrian ¹⁾.

"Hallucigenia refers to the bizarre and dream-like appearance of animal" ¹⁾, as its name suggests, Hallucigenia has an enigmatic form. Hallucigenia is a member of the "lobopodans", a group of ancient worms which possess pairs of leg-like extensions of body. The affinities of this group are controversial, may be a base group of Panarthropods or a stem group to modern Onychophorans (velvet worm) ²⁾.

The length of Hallucigenia is between 0.5 and 3.0 cm ¹⁾, relatively small.

Hallucigenia has a worm-like body with 7 pairs of dorsal spines and also 7 pairs of slender legs terminated in a pair of claws. The neck has 2 or 3 pairs of slightly short appendages ²⁾. The elongated head with a pair of simple eyes terminates with a mouth which has a radial array of aclerotized elements resemble to ones in targigrades ⁴⁾.

Hallucigenia may have fed on sponges, or scavenged on decaying animal remains ²⁾.

Hallucigenia was described as a genus of the Annelids (Polychaeta), but afterwards, it has redescibed and redifined as a new genus and a new family, not an annelid ¹⁾. Subsequently, it has been found that the not antero-posterior but also dorso-ventral orientations of Hallucigenia proposed until then are reversed ²⁾.

Thanahita distos, a species of the family Hallucigeniidae and a distinct genus from Hallucigenia, found from the Silurian, suggesting that this clade would have survived to the Silurian ³⁾.