Which was one of the first animals to develop a true body cavity?

Coelomates are animals which body cavity (coelom) is filled with fluid. Fluid surrounds organs, protects them and allows their free movement and growth. All vertebrates and some invertebrates, such as mollusks, annelids, and arthropods, are coelomates.

science-ocean floors and habitats study guide by volleyballqueen3817 a major advance in ocean-floor mapping. list four sections of the ocean floor. algae that use sunlight to produce their own food through photosynthesis. all of the feeding relationships that exist in a habitat. drying out, predators, find food.