Which of the following terms includes organisms with only two germ layers?

The term "didermic" refers specifically to organisms that possess only two germ layers: the ectoderm and the endoderm. This classification is important in the study of embryonic development and the organization of tissues in different types of organisms. Organisms with two germ layers, such as cnidarians (including jellyfish and corals), exhibit a simpler level of body organization compared to those that develop three germ layers.

The correct understanding of the term is crucial for distinguishing between different types of animals based on their embryonic development. In contrast, "endodermic" typically relates to the inner germ layer but does not define the presence of only two layers. "Exodermic" is not a standard term in developmental biology and does not accurately describe any known developmental stage or classification. The term "tri-layered" refers to organisms with three germ layers—ectoderm, mesoderm, and endoderm—further emphasizing that "didermic" is indeed the correct term for those with two germ layers.