Hallucigenia is a possible invertebrate from the Early to Middle Cambrian period of Canada and China.
Hallucigenia grows 0.5 to 3 centimeters long and has a wormlike body, with many specimens exhibiting a poorly defined "blob" at one end. The other end of the body appears to taper into a tail-like structure. Pairs of sharp, spiny appendages lined the end of the animal commonly thought of as the back, while corresponding pairs of tentacles lined the opposite side.
Although there are interpretations that have gained support within the scientific community, it is still uncertain which end is the "head" and which is the "tail".
It is believed that Hallucigenia is distantly related to modern arthropods, or possibly velvet worms. Some scientists believe that it is related to the much larger Anomalocaris, which has been found in the same time and place. There are three known species: H. sparsa (the type species), H. fortis, and H. hongmeia (described in 2012).
Hallucigenia was first described in 1977 by Simon Conway Morris from the Burgess Shale deposits in British Coulmbia, Canada. He named it because of its "bizzare and dreamlike quality", referring to the unfamiliarity of the animal. Conway Morris interpreted the blobular end of Hallucigenia as the head, and hypothesized that the animal walked on the spiny appendages, using the tentacles on its back for feeding purposes. Although this depiction raised questions, it was the best available one at the time.
In 1991, Lars Ramskold and Hua Xianguang reclassified Hallucigenia as a type of primitive velvet worm, based on their work on a similar animal, Microdictyon, in China. They found that Conway Morris's restoration was in fact upside down: the spines rested on Hallucigenia's back, and did not act as legs. They also discovered that the blobular "head" found on many specimens was in fact a stain, and not an actual part of the body. This interpretation is not definite, but it is a much better explanation than was put through by Conway Morris.
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