Their tentacles mark belemnites, squids, nautiluses, octopuses and cuttlefishes as cephalopds. Cephalopods in turn belong to a larger group of invertebrates called mollusks. In cephalopods, this foot is divided into several long arms or tentacles that lie ahead of the eyes and surround the mouth. Like snails, cephalopods have tooth-like radula. But they also have parrot-like jaws used for biting.
Cephalopods are especially impressive for their advanced brains and sophisticated eyes.
The earliest marine invertebrates were mostly slow-moving animals. Like modern clams, snails, and jellyfish, they crawled or burrowed in the seafloor, or floated lazily with the currents. About the time of the Early Ordovician Period, there was an explosion of suspension feeders animals that floated or swam above the seafloor. Their predators followed. Nautiloids were pioneer cephalopod mollusks.