Nonbilaterian

Nonbilaterian refers to animals that do not exhibit bilateral symmetry and typically lack a distinct anterior-posterior axis. The term covers the four traditionally recognized phyla: Porifera (sponges), Cnidaria (jellyfish, corals, sea anemones), Ctenophora (comb jellies), and Placozoa. Some phylogenetic frameworks place Xenacoelomorpha near the base of the bilaterian lineage, which affects whether they are classified as nonbilaterian or early bilaterians; accordingly, the scope of the term can vary.

Porifera are simple, sessile filter feeders that lack true tissues, organs, nerves, and muscles. Their bodies are built around a canal system lined with choanocytes, and they often produce a skeleton of spicules or collagen-like fibers (spongin). Sponges can have irregular shapes and display a range of pore-rich forms, but they do not form the organized tissue layers characteristic of other animals.

Cnidaria and Ctenophora are generally radially symmetric and diploblastic, possessing two primary tissue layers. Cnidarians include corals, jellyfish, and sea anemones and typically feature tentacles with stinging cells and a gastrovascular cavity. Ctenophores, or comb jellies, are free-swimming predators with rows of ciliary plates. Both groups have nervous systems organized as nerve nets rather than a centralized brain.

Placozoa are among the simplest known multicellular animals. They have a very small, flat body composed of a few cell types arranged in two cellular layers, with no true tissues or organs. Placozoans lack a nervous system and gut; they feed by external digestion and move by gliding.

Nonbilaterians provide insight into early animal evolution, representing lineages that diverged before the evolution of bilateral symmetry. They raise questions about the origin of tissues, nervous systems, and complex organs, and remain subjects of ongoing debate in systematics and paleontology.