The Two Spot Octopus that reside in California waters are members of the invertebrate phylum Mollusca, which translates to soft body. The octopus is true to their phylum in that they indeed are soft, with only a beak as the hard part of their body. Octopus belong in the class Cephalopod, which means head foot and other members of the class include squid, nautilus and cuttlefish.
Many people believe that octopus move only by pushing with their arms and tentacles but they can move via jet propulsion and only a few animals in the ocean can do that. Jet propulsion happens when the animal takes in water through its mantle. The water is then pushed through the siphon, a tube like part that has a larger opening on the inside and becomes narrower. This is comparable to putting your thumb on the end of a hose in an attempt to get the water to go further. With this method of travel, some Cephalopod can move in excess of 25 miles per hour. Furthermore, the water being pushed out the siphon can be used to disburse ink, creating a smoke screen that can help them evade predators.
http://nationalzoo.si.edu/animals/invertebrates/facts/cephalopods/locomotion.cfm
http://jrscience.wcp.miamioh.edu/fieldcourses03/PapersMarineEcologyArticles/THEJETSET.THEANATOMYOFSWIA.html
