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Phylum is a hierarchical method of grouping animals within a particular group based on their traits. The hierarchical classification is very essential as it enables us to identify traits easily. The phylum is a rank that is present below the kingdom.

Echinoderms are successful animals that exhibit bilateral symmetry during the larval stages. These organisms possess two coelomic cavities. The first coelom during the later stages protects the internal organs, and the second coelom produces the vascular system.

Thus, Echinoderms are bilaterally symmetrical in the larval stage, and they possess a coelom.

Therefore, the given option is correct.

Nematodes are called roundworms because their body is tubular. These organisms have distinct mouths and anus. They are called pseudocoelomates because the coelom within the body is made of endoderm on one side and mesoderm on the other.

Thus, Nematodes are called roundworms, and they have a pseudocoelom.

Therefore, the given option is correct.

Platyhelminthes are referred to as flatworms due to their body type. There is an absence of body cavity and hence are referred to as acoelomates. The digestive tract is comparatively very simple. Therefore, they possess a gastrovascular cavity from where food is taken, and wastes are eliminated.

Thus, Platyhelminthes are called flatworms, and they do not have a coelom.

Therefore, the given option is correct.

The organisms belonging to the phylum Porifera, such as the sponges, are acoelomate organisms that do not have a body cavity due to the absence of tissues. These organisms also lack a gastrovascular cavity because this feature is only observed in Coelenterates and Cnidarians.

Thus, the organisms in phylum Porifera are acoelomates, and they lack a gastrovascular cavity.

Therefore, the given option is incorrect.