The animal itself was segmented and soft shelled. The head carried an array of five apparently functional eyes and a long flexible snout that appears to be in no way homologous to the appendages on the head of any other lifeform of the time. The body segments each featured a set of gills and a pair of flap like appendages dissimilar to other animals of the time. The rearmost three flaps formed the tail. There were spines associated with the snout and tail. Unlike known arthropods, the head appears not to be formed from fused segments. The animal was covered with what appears to be a soft, flexible shell with no joints between the segments. Opabinia has no known relatives except possibly Anomalocaris.
Although Opabinia is a relatively minor constituent of the early faunas it has historical significance because it was one of the first truly unusual animals to be completely studied and described when redescription of the Burgess shale faunas was undertaken in the 1970s. Harry Whittington showed pretty convincingly in 1975 that the animal, previously thought to be an arthropod, was not an arthropod and moreover that it was unlikely that it belonged to any other known phylum. Taken with two other unexpectedly unique arthropods Marella and Yohoia that had been previously been described, Opabinia demonstrated that the softbodied Burgess fauna's were much more complex and diverse than anyone had suspected.