Cephalopods

Cephalopods are a small class of mollusks arguably containing the most intelligent of all the invertebrates. There are approximately 650 living species, ranging from the tiny paper nautilus to the giant squid.

The name "Cephalopoda" literally means "head-foot", as the foot is directly attached to the head. This is one of several characteristics that most members of this class share.

The most noticeable characteristic is the tentacles. Ranging in number from 8 to 90 depending on the species, these tentacles form a crown around the head and are specialized for grasping prey. These tentacles are specialized feet, and are sometimes equipped with suckers for extra grip. These suckers may be further equipped with hooks.

This class has a very specialized foot known as the siphon, or hyponome, and enables them to locomote. Water is drawn into the mantle cavity and then forcibly expelled through the siphon, creating a jet propulsion that shoots them forward, backward, up, down, side to side, depending on what direction the siphon is pointed.

Most cephalopods have a chambered shell, the most notable example being the nautilus with its large, snail-like shell. Other cephalopods, such as squid or cuttlefish, have a smaller, internal shell. Octopods lack shells entirely.

The mouth of a cephalopod is composed of a substance known as chitin, and is shaped like a parrot's beak. The tongue is rough and is known as the radula, and draws food into the mouth.

Cephalopods are the most complex of all the invertebrates, and for a good reason: many of them are huge. While the majority are only a few feet in length, the giant squid reaches an amazing length of 60 ft (18 m), and is the largest of all the invertebrates. Thus, they require an advanced nervous system. Most invertebrates have clusters of nerve cells known as ganglia which act as a simple brain. In cephalopods, the ganglia are concentrated and fused together to form a brain. This brain is similar to that of a vertebrate. It is separated into different sections that control separate areas of the body. For example, forward swimming and the closing of the suckers are controlled by the cerebral ganglia.

Cephalopods have three hearts in a mostly-closed circulatory system that pumps blue-coloured blood throughout the entire body. The senses are acute, with the eyes of the giant squid being the largest in the animal kingdom. These eyes can sense light and can form images, which aid with hunting as cephalopods are highly carnivorous. The giant squid has even been known to attack sperm whales!

Cephalopods have several modes of defence. First, they can ward off attackers with their tentacles. Secondly, chromatophores in their cells allow them to change colour, with some species rivalling the chameleon for camouflage. Lastly, most species have a sac which contains sepia, a black ink-like substance that can envelop and temporarily cloud the enemies' vision.

Cephalopods are found throughout all of the world's oceans, from the cold Arctic to the warm tropics to the dark levels of the abyss. They are strictly marine, and there are no terrestrial species.

The taxonomy of cephalopods is still being debated. One of the many proposed methods is the 5-order system, with a subclass for nautiluses:

Suborder Coleoidea
Belemnoidea (extinct belemnites)
Octopoda (octopods, argonauts) 250 spp
Sepiida (cuttlefish)
Sepiolida (small cuttlefish-like cephalopods)
Teuthida (or Teuthoida, Tenthoidea) (squid) 266 spp
Vampyromorphida (deep-sea vampire squid)
Subclass Nautiloidea (nautiluses – no orders, just 1 family) 7 spp
Subclass Ammonoidea (ammonites – extinct cephs)

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