Cephalopods were formerly supposed to have developed from a monoplacophoran-like progenitor with a bent, tapering shell, and they were thought to be closely related to gastropods (snails). The early shelled cephalopod Plectronoceras's resemblance to some gastropods was used to support this theory. However, more recently this view has been challenged by researchers who argue that no such relationship exists because of significant differences between cephalopods and gastropods.
In fact, there are now several lines of evidence showing that cephalopods are more closely related to each other than either group is to gastropods. One study concluded that cephalopods are more closely related to arthropods (including insects) than they are to gastropods. Another study found that cephalopods share certain features with cubozoa (a group of jellyfish-like organisms) suggesting that both should be placed in a separate group called Cnidaria. A third study showed that squid use a form of oxygen-carrying protein similar to hemoglobin found only in vertebrates (not invertebrates such as squid). This evidence indicates that cephalopods are more closely related to vertebrates than they are to gastropods, which do not contain proteins with vertebrate blood types such as alpha or beta globulins.
The common octopus, Octopus vulgaris, was originally described by Linnaeus in 1758.
While the other cephalopods, like their mollusc cousins, have an inner or outer shell, the octopus does not. The term cephalopod literally means "head-footed," referring to the fact that these creatures' limbs sprout straight from their heads. However, they do not have any skin between their bodies and their shells; instead, they contain a large internal muscle layer.
Unlike most other animals, which have two eyes on the front of their head, the octopus has eight eyes distributed throughout its body. These eyes are very sensitive to changes in light intensity and color, allowing the octopus to see better in dark caves and under water than any other animal. Also, because they cannot move them, these eyes give the octopus a huge advantage over other creatures when foraging for food in deep waters where sunlight does not reach.
Although they lack a face, octopuses do have several distinct anatomical features that serve a similar function. Just as humans have eyebrows and eyelids, so too do octopuses. They also have a small nose, slightly protruding lips, and a chin. These features help the octopus navigate through its environment by sensing chemical traces in the air. They also communicate with each other using chemicals released into the water from special glands located around their bodies.
Octopuses are one of the few invertebrates that can regenerate their limbs.
Mollusks classified as cephalopods include the pearly chambered Nautilus, squids, and octopus. All have similar shells composed of multiple compartments containing soft tissues: muscles, nerves, stomachs, intestines, and blood vessels.
They are all mollusks, members of the phylum Mollusca. The word "mollusk" comes from the Latin word mola, meaning "moon," because of the shape of some species' shells.
Octopuses are one of the eight genera of cephalopod. They are unique among cephalopods in that they use ink as their weapon. In addition, they use their tentacles like hands to manipulate objects during hunting and camouflage.
Ink is a mixture of chemicals secreted by special cells called chromatophores. It is used as a defense mechanism and as a means of communication. When an octopus is threatened, it expands or contracts certain muscles to force out more of the fluids that make up its ink.
This behavior is similar to that of a seal when it sprays water from its nose to scare off predators. However, an octopus can control the direction of its spray using its head while a seal cannot.