Once again we have echinoderms. Echinoderms are absolutely exclusive to the ocean and have no fresh-water or terrestrial based representatives. As for a possible reason why they are exclusive to marine habitats I would theorize that simply nothing of their relatives ever adapted towards land. Based on the theory that all life probably began deep within the ocean, even what life is now on land has ancient origins from the ocean. Though despite this there are still exclusive marine species because it seemed these life forms just never left the ocean. While many land creatures are amazing organisms in their own right, especially with the origins of starting out as a microorganism, to a sea dwelling creature then somehow over many years finally
