T he cephalopods are an ancient and very successful group, some of which are now the most advanced of all the invertebrates. They have long been among the dominant large predators in our oceans. The Tetrabranchia possess an external shell and two pairs of gills. On the whole cephalopods are adapted for a free-swimming existence. Some cephalopods such as the octopus, however, have largely given this up for a less active bottom dwelling life, although many of these are active hunters and scavengers.
Cephalopods are the most active of all the molluscs and can even rival the fish for swimming speeds.
Although there are relatively few species of living cephalopods, they inhabit a great variety of habitats in all the world's oceans. Most cephalopods have a sac that produces brown or purplish-black ink used to ward off attacking fish or to serve as a protective smoke screen. Cephalopods have attained the largest size of all the molluscs. Individual species, especially the squid, are often very abundant and provide major targets for marine fisheries. The chalky cuttlefish bone is also used for a calcium supplement for birds and at time used in making toothpaste.