<Tr> <Td> </Td> <Td> <P> Strongylocentrotidae </P> </Td> </Tr> <Tr> <Td> 450 mya </Td> </Tr> <P> Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines . These are a common source of injury to ocean swimmers, especially along coastal surfaces where coral with stationary sea urchins are present . Their stings vary in severity depending on the species . Their spines can be venomous or cause infection . Granuloma and staining of the skin from the natural dye inside the sea urchin can also occur . Breathing problems may indicate a serious reaction to toxins in the sea urchin . They inflict a painful wound when they penetrate human skin, but are not themselves dangerous if fully removed promptly; if left in the skin, further problems may occur . </P> <P> Sea urchins are traditional model organisms in developmental biology . This use originated in the 1800s, when their embryonic development became easily viewed by microscopy . The transparency of the urchin's eggs enabled them to be used to observe that sperm cells actually fertilize ova . They continue to be used for embryonic studies, as prenatal development continues to seek testing for fatal diseases . Sea urchins are being used in longevity studies for comparison between the young and old of the species, particularly for their ability to regenerate tissue as needed . Scientists at the University of St Andrews have discovered a genetic sequence, the' 2A' region, in sea urchins previously thought to have belonged only to viruses that afflict humans like foot - and - mouth disease virus . More recently, Eric H. Davidson and Roy John Britten argued for the use of urchins as a model organism due to their easy availability, high fecundity, and long lifespan . Beyond embryology, urchins provide an opportunity to research cis - regulatory elements . Oceanography has taken in interest in monitoring the health of urchins and their populations as a way to assess overall ocean acidification, temperatures, and ecological impacts . </P>

What is the edible part of a sea urchin