What characteristic do Cnidarians and Ctenophores share?

Cnidarians and Ctenophores are both classified as diploblastic organisms, meaning they develop from two embryonic tissue layers: the ectoderm and endoderm. This characteristic is foundational in the animal kingdom, distinguishing diploblasts from triploblastic organisms, which possess a third layer called the mesoderm. The presence of only these two layers in both Cnidarians (like jellyfish and corals) and Ctenophores (commonly known as comb jellies) suggests a simpler level of body organization that reflects their evolutionary relationships.

In contrast to this, the other choices do not accurately depict a shared characteristic between these two groups. The idea of alternating between bilateral and radial forms does not apply to either group as they exhibit predominantly radial symmetry throughout their life cycles. Some members may have supportive structures, but this is not a universal characteristic that applies to both Cnidarians and Ctenophores in a consistent manner. Moreover, while some individuals within these groups may exhibit scavenging or parasitic behavior, it is not a defining characteristic that is universally applicable to all members of either group. Thus, the shared attribute of being diploblastic best encapsulates the fundamental similarity between Cnidarians and Ctenop