A cephalopod is a member of the class of animals known as Mollusca or mollusks. They are a phylum within that class that involves animals with distinctive features such as noticeable heads, some sort of arms or tentacles for grasping and eating, and bodies that can be divided down the middle and show symmetry on both sides. Though they are classified as being closely related to mollusks, slugs, and snails, they are the most advanced phylum in the Mollusca group. Cephalopods are the largest known invertebrates, meaning that they lack a backbone, and common members of the group include octopuses, squid, and cuttlefish.
The Mollusca class as a whole is known to contain at least 55,400 individual species, but the cephalopod is a minority in this group, with squid species only numbering 650 and total cephalopod species totaling 800. Their advanced development compared to other members of Mollusca makes them appear more like vertebrates than their closest relatives. Only one type of cephalopod, in fact, has an external shell, which is the Nautilus, and all the others are free-swimming animals. They are often labeled as inkfish by local fishermen who reside in coastal regions that they inhabit, as they have the ability to expel a cloud of inky material as a distraction and cover to escape predators.
The Kraken is a legendary form of cephalopod, and sailing ships of the 18th century described it as being a giant squid the size of a small island at least a mile and a half across (2.4 kilometers). They swore that it could reach the top of a ship's mast with its tentacles fully extended. The Kraken would supposedly sink ships by wrapping its tentacled arms around one and capsizing it. The origin of the Kraken legend is likely found in Scandinavian waters, where local stories claimed it drew large numbers of fish to its presence and could be a boon to local fishermen if not disturbed.
Giant squid are known to be real animals, though their size is not what legend would have one believe. They are projected to be large enough, however, that they could be predators for creatures such as the sperm whale. This may be the reason that they have attacked ships, mistaking them for whales. There are three documented cases of fearless giant squid attacking ships in the 1930s, where they probably died when pulled into the ships' propellers. One that washed ashore in New Zealand in 1933 measured 66 feet (22 meters) in length and weighed several tons (several thousand kilograms).
The study of cephalopod species is known as teuthology, and it has revealed some surprising facts over the years. They are known to exist in every ocean regardless of whether it is warm and tropical or a freezing polar region. They can live near the surface or down in the depths of the water. They are believed to have once been the dominant form of life in the oceans of the world, and have been around since before even the most primitive of fish emerged and before plant life existed on land. The intelligence of octopi is also being researched, as they are now known to play and have distinctive personalities, which only the most advanced of sea creatures do.