sentences:  Importantly, urchins act as the closest living relative to chordates and thus are of interest for the light they can shed on the evolution of vertebrates.The genome of "Strongylocentrotus purpuratus", was completed in 2006 and established homology between sea urchin and vertebrate immune system-related genes.Sea urchins code for at least 222 Toll-like receptor genes and over 200 genes related to the Nod-like-receptor family found in vertebrates.