Which phylum is described as the most primitive among triploblasts and has organs?

Which phylum is described as the most primitive among triploblasts and has organs?

• A. Platyhelminthes
• B. Nematoda
• C. Annelida
• D. Cnidaria

Answer: (A)

The idea here is about body plans in relation to germ layers and body cavities. Platyhelminthes are triploblastic, meaning they develop from three germ layers, and they have organ-level organization. However, they lack a true body cavity (they are acoelomates). This combination—three germ layers with organs but no coelom—marks a very early, simplest body plan among triploblastic animals. In contrast, cnidarians are diploblastic (two germ layers), nematodes have a pseudocoelom (a false cavity), and annelids are coelomates with a true, segmentally organized body cavity. Therefore, this phylum fits as the most primitive among triploblasts that still possess organs.

The idea here is about body plans in relation to germ layers and body cavities. Platyhelminthes are triploblastic, meaning they develop from three germ layers, and they have organ-level organization. However, they lack a true body cavity (they are acoelomates). This combination—three germ layers with organs but no coelom—marks a very early, simplest body plan among triploblastic animals. In contrast, cnidarians are diploblastic (two germ layers), nematodes have a pseudocoelom (a false cavity), and annelids are coelomates with a true, segmentally organized body cavity. Therefore, this phylum fits as the most primitive among triploblasts that still possess organs.