Hallucigenia sparsa is the Middle Cambrian animal discovered from the Burgess Shale, has a slender tubular body as a whole ^{1), 2), 3)}. Between the slightly rounded head and the long trunk is a narrow neck ^{2), 3)}. The body length is approximately 3 cm ²⁾.

The head has a mouth opening at the anterior end and a pair of eyes (simple visual organs, not compound eyes) on the dorsal side ³⁾. Three pairs of appendages on the neck are nearly equal length with the fourth through seventh appendages, but thinner than the others ³⁾.

The trunk has seven pairs of appendages bearing terminal claws ^{2), 3)}. The last pair of appendages protrude from the posterior end of the body and extend obliquely ^{2), 3)}. The dorsal spines are located in dorsal side to each position of the third through ninth appendage pairs ^{2), 3)}.

Presumably, Hallucigenia lived on the seafloor or on the surface of sponges, preying on those sponges, or was a scavenger, feeding on dead animals ²⁾.

Hallucigenia is considered a stem group of the modern onychophoran, but its exact phylogenetic position has not yet been determined ^{2), 3)}.

Since Hallucigenia sparsa has a bizarre shape and its available specimens are rare, generic name "Hallucigenia" is derived from the Latin "hallucinatio" (wandering in the mind) and specific name "sparsa" from "sparsu" (rare) ²⁾.

Hallucigenia was described at the first time in 1911 as the annelid polichelata by Walcott who discovered many of the Cambrian animal fossils ^{1), 2)}. After that, it has re-described by Conway Morris, but its relationship was still unknown and the anatomical interpretations on both of the dorso-ventral and the antero-posterior orientations were reversed ²⁾.