In which of the following phylum, would you place an animal with a bilaterally symmetrical, triploblastic body without a true internal cavity (coelom) ?

The animal described belongs to the phylum Platyhelminthes. These organisms, commonly known as flatworms, are characterized by bilateral symmetry and a triploblastic body plan, meaning they develop from three embryonic germ layers: ectoderm, mesoderm, and endoderm. Crucially, they are acoelomate, meaning they lack a true internal body cavity (coelom) between the digestive tract and the outer body wall [1]. In contrast, Porifera (sponges) are generally asymmetrical and lack true tissues. Cnidarians exhibit radial symmetry and are diploblastic, possessing only two germ layers. Annelids, while bilaterally symmetrical and triploblastic, are coelomates, possessing a well-developed segmented body with a true internal cavity. Therefore, the combination of bilateral symmetry, triploblastic construction, and the absence of a coelom uniquely identifies Platyhelminthes.

Sources

1. [1] https://www.sciencedirect.com/topics/medicine-and-dentistry/platyhelminth