Final answer:
The previously unknown animal described in the question should be classified under Phylum Platyhelminthes, as it shares critical characteristics such as being triploblastic, acoelomate, dorsoventrally flattened, and having flame cells with flatworms of that phylum.
Step-by-step explanation:
The animal described is an invertebrate, triploblastic, dorsoventrally flattened, acoelomate, and has flame cells. Given these characteristics, the correct phylum to place this animal in would be Phylum Platyhelminthes. Flatworms, which are part of this phylum, are triploblastic and acoelomate, meaning they have three germ layers and no coelom, respectively. The dorsoventrally flattened shape is typical of flatworms, and the presence of flame cells is indicative of the flatworms' rudimentary excretory system. Therefore, based on the provided characteristics, the animal would indeed be classified under Phylum Platyhelminthes