Classical studies in developmental biology were often making use of the abundancy of live eggs and embryos of marine organisms. Pioneering studies in sea urchins and sea squirts have paved the way to fundamental biological concepts. The advent of molecular techniques as well as modern imaging techniques has further made such models a corner stone of modern approaches in developmental biology but also in marine biology. Moreover, the diversity of different animal species and phyla allow direct comparison of mechanisms underlying developmental processes and pathways and thus are link to evolution in a field often referred to as Evo-Devo.