Nearly all are filter feeders. Mussels are also sessile using a thread-like material to anchor themselves to roots of plants or other surfaces. Scallops sit on top of the substrate, and are able to swim away from predators. The cephalopods, which include the squid, octopus and nautilus, possess the most highly developed nervous systems and are the largest of all invertebrates. Most cephalopods utilize jet propulsion to swim by expelling water from the body cavity through a siphon.
Cephalopods are predators and use their tentacles to capture and manipulate prey.
Squid possess ten tentacles whereas octopus have eight. They both possess a parrot-like beak and inject a poison to subdue prey. They are found only in marine waters and most are bottom dwellers. Their bodies are divided into five parts arranged in a disc, sphere or cylinder. Most are scavengers or carnivores and feed on worms, mollusks, corals, sponges, crustaceans and even fish.