As of 2015, scientists believe the first animal on Earth was the comb jelly of the phylum Ctenophora. It is an ocean-dwelling, gelatinous organism whose length varies from less than half an inch to 16 inches. Until 2008, sponges of the phylum Porifera were considered the first animal.
Comb jellies have been in existence for over 500 million years and were the first animals to use their muscles for swimming. Jellies consist of 95 percent water, and their structural proteins, muscle and nerve cells form an internal skeleton. Jellies have a primitive nervous system called the nerve net, but they have no stomach, intestines or lungs. They are named after their eight plates of fused cilia called combs, which help in locomotion. Most jellies have a single pair of tentacles with specialized cells to catch their prey.