Phylum Mollusca (5)
This class includes the nautilus, octopus, squid, and cuttlefish. The shelled cephalopods are related to the modern nautilus and have a single shell that coils in one plane. Septa divide the shell into chambers. A tube called a siphuncle connects the last-formed chamber with previous ones. The body of the animal occupies the last chamber; the rest are filled with gas, which controls the animal's buoyancy in the water. The foot is modified into a set of tentacles for grasping prey. Cephalopods have a well-developed eye. A funnel for jet propulsion allows the cephalopod to swim in any direction.