A squid or an octopus. A better answer for this question would be: They are an invertebrate in the phylum Mollusca meaning "soft bodied" (Mollusks). The class Cephalopoda means "head foot", meaning...
cephalopod (a class of mollusc characterized by arms or tentacles and a prominent head)
A cephalopod is a snail.
by using jet propulsion
I don't know, but cephalopods are octopi, squid, cuttlefish and nautili.