It is just common sense to avoid a sharp black spiny thing such as sea urchin. Sea urchin spines can penetrate deeply into the flesh and break off. This usually causes long-lasting inflammation if not surgically removed. There is doubt, though, as to whether the spine carry venom or not.
Most sea urchins are either scavengers or vegetarians. They feed on either plant material or small animals on the seafloor. Many sea urchins have five sharp teeth for scraping food off the substrate. The heart urchins and their relatives do not have teeth and dig in sand or mud to get their organic material for food. Small pincerlike organs, the pedicellariae, lie between the spines and serve many functions, including cleaning the test and scaring off predators. Some of these pedicellariae are venomous to humans. The roe, or scentifically gonads, are considered a delicacy in the Orient and in some parts of South America.
A less common but much more dangerous urchin is the flower urchin. Instead of long spines it appears to be covered with numerous flowers which are in fact little venomous pincers (pedicillariae) capable of causing paralysis and even death. It has killed several people in Japan.