The Cephalopods ("head-foot") are the mollusk class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. Teuthology, a branch of malacology, is the study of cephalopods.
The class contains two extant subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopuses, squids, and cuttlefish. In the Nautiloidea the shell remains; this subclass includes the nautilus. There are around 786 distinct living species of Cephalopods. Two important extinct subclasses are Ammonoidea, the ammonites and Belemnoidea, the belemnites.
Cephalopods are found in all the oceans of the world and at all depths. None of them can tolerate freshwater, but some few species do tolerate more or less brackish water. They are regarded as the most intelligent of the invertebrates and have well developed senses and large brains; larger than the brains of gastropods or bivalves. With the exception of Nautilus, they have special skin cells called chromatophores that change color and are used for communication and camouflage. The nervous system of cephalopods is the most complex of the invertebrates. The giant nerve fibers of the cephalopod mantle have been a favorite experimental material of neurophysiologists for many years. Many species can see polarization of light. They are probably colorblind, yet they distinguish a vast number of tones.
Cephalopods' primary method of movement is by jet propulsion, a very energy-consuming way to travel compared to the tail propulsion used by fish. The relative inefficiency of jet propulsion worsens with larger animals. This is probably the reason why many species prefer to use their fins or arms for locomotion if possible. Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.
Some octopus species are also able to walk along the sea bed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of muscle around the mantle.
With a few exceptions, Coleoidea live by the motto "live fast, die young". Most of the energy extracted from their food is used for growing. The penis in most male Coleoidea is a long and muscular end of the gonoduct used to transfer spermatophores to a modified arm called a hectocotylus. That in turn is used to transfer the spermatophores to the female. In species where the hectocotylus is missing, the penis is long and able to extend beyond the mantle cavity and transfers the spermatophores directly to the female. They tend towards a semelparous reproduction strategy; they lay many small eggs in one batch and die afterwards. The Nautiloidea, on the other hand, stick to iteroparity; they produce a few large eggs in each batch and live for a long time.
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