Description

Hallucigenia belongs to the lobopodian group and is recognized from fossils found in Cambrian-era deposits, similar to those found in the Burgess Shale, in locations spanning Canada and China. Isolated spines attributed to Hallucigenia have also been discovered in various parts of the world. The choice of its generic name is reflective of the peculiar appearance of the type species and the rather unusual history of its study. When initially categorized as a genus, H. sparsa was depicted as an enigmatic creature, oriented in an upside-down and back-to-front manner.

Lobopodians represent a stage in the evolution of Paleozoic panarthropods, from which the lineage of velvet worms, water bears, and arthropods eventually emerged. Hallucigenia takes the form of a long, tubular organism with as many as ten pairs of slender legs known as lobopods. The initial 2 or 3 pairs of legs are slim and lack distinctive features, while the subsequent 7 or 8 pairs conclude with either one or two claws. Positioned above the body's trunk are seven pairs of rigid conical sclerites, corresponding to the 3rd through 9th leg pairs. The trunk can either lack distinct features, as seen in H. sparsa, or be divided by varying annulations, as observed in H. fortis and H. hongmeia. Identifying the "head" and "tail" of the creature is a challenge, as one end extends beyond the legs, often drooping down as if reaching toward the substrate. Some specimens exhibit traces of a simple gut. Research conducted in the mid-2010s clarified that the longer end functions as the head, housing an anteroventral mouth and at least a pair of simple eyes.