Which phylum is primarily composed of marine organisms and has bilateral symmetry?

The correct answer is based on the characteristics of the phylum Platyhelminthes, commonly known as flatworms. This phylum includes various species, some of which are indeed marine organisms. Members of Platyhelminthes exhibit bilateral symmetry, meaning their body can be divided into two equal halves along a single plane. This symmetry is often a significant feature in the organization of body structure and is crucial for the functioning of their nervous systems, allowing for a more directed movement and feeding behavior.

While Chordata includes marine organisms and also displays bilateral symmetry, it encompasses a much broader group that includes all vertebrates and some invertebrates, rather than being primarily marine. Nematoda (roundworms) also shows bilateral symmetry and includes many terrestrial species, not just marine. Echinodermata, on the other hand, primarily consists of marine organisms such as starfish and sea urchins, but they typically exhibit radial symmetry as adults, which distinguishes them from those that maintain bilateral symmetry throughout their life cycle. Thus, Platyhelminthes stands out as the phylum that meets both criteria of being primarily composed of marine organisms and exhibiting bilateral symmetry.