Marine invertebrate species have been used as model organisms in evo-devo studies for over a century. These species provided a great advantage in seminal microscopic studies because of the large size of their eggs and embryos and the easy accessibility to the first embryological processes afforded by their external fertilization and development. This review provides a historical perspective on the use of marine invertebrates—including echinoderms, ascidians, and spiralians—in the study of embryo mechanics. Here, we highlight the key contributions of marine invertebrates to the understanding of cortical and cytoplasmic mechanics, the implementation of early cleavage patterns, and tissue mechanics. We also examine the emergence of different blastula shapes in metazoans and focus on the clear dichotomy between compact and hollow embryos, suggesting a canalization of a compact embryo shape in taxa that display invariant cleavage patterns such as nematodes, spiralians, and ascidians. With recent advances in high-resolution imaging, computational modeling, and the development of modern genetic and genomic tools, marine invertebrate model organisms continue to be at the forefront of evolutionary developmental biology and mechanobiology. Their contribution to these fields not only provides invaluable insights into the fundamental principles of morphogenesis but also offers an ideal comparative framework that allows the exploration of the evolution of mechanical and biological processes across metazoans.