Nerves, muscles and true tissues split most animals from the humble sponge
Nearly every animal, from jellyfish to humans, belongs to a proposed group called Eumetazoa, meaning roughly true animals. What unites them is organised tissue, nerve-like cells and, in many cases, the ability to move. Sponges sit outside, and a surprisingly heated scientific argument continues over whether comb jellies do too.
The name blends Greek for well, after and animal; alternative names are Epitheliozoa and Histozoa. The standard picture places sponges, phylum Porifera, as the sister to everything else. Eumetazoa then splits into comb jellies, the Ctenophora, and a far larger branch called ParaHoxozoa that includes jellyfish and corals, the tiny flat placozoans, and the bilaterians, animals with left and right sides, which cover most familiar creatures.
Most members share true tissues arranged in germ layers, nerves and muscles, and an embryo that passes through a stage called the gastrula. In the Five Kingdoms scheme of Lynn Margulis and K. V. Schwartz, the group meant simply all animals except sponges. Some researchers once wondered whether sponges and other animals had separate single-celled ancestors, which would break the animal kingdom apart. Genetic studies, along with collar cells called choanocytes, have since settled that animals share one origin.
The rival Myriazoa hypothesis flips the early branching, putting comb jellies first and grouping sponges with the rest. If true, either sponges lost their complexity or comb jellies evolved nerves and muscles independently. As of May 2026 the question remains open, and the dispute has grown combative enough that some scientists have chosen to stay out of it.
Dating the group is equally slippery. One molecular clock method and one reading of fossils place its origin in the Ediacaran period, but early members may have left little trace. Vernanimalcula, claimed as a tiny three-layered animal from the end of the Marinoan glaciation, would push the date earlier. Only a few Ediacaran fossils, including Kimberella, Dickinsonia and Haootia, are firmly accepted as eumetazoans, because those impressions preserve so little detail.
Source: Eumetazoa
