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Some of the most innovatively adapted invertebrates belong to class Cephalopoda, Greek for “head-footed.” Over 700 species of cephalopods have been identified, and they are divided into the subclasses Coleoidea (cuttlefish, squid, and octopus) and Nautiloidea (nautiluses).

Cephalopods inhabit all of the world’s oceans and occur at a wide range of depths, from ocean bottoms (benthic species) to open waters (pelagic species). They are also diverse in size, ranging from the centimeters-long Californian octopus (Octopus micropyrsus) to giant squid (Architeuthis), some of which measure over 18 meters long.

Although cephalopod species have many differences among them, they all share several common features:

Habitat
All cephalopods inhabit marine environments. There are no freshwater cephalopod species.

Diet
Cephalopods are strictly carnivorous. They all possess a hard beak used for defense and tearing prey. Most species hunt for prey, while some are scavengers.

Life Span
Generally, cephalopods grow quickly and have short life spans. Most live from one to two years, with the exception of the nautilus, which may live more than 15 years.

Body Plan
The basic cephalopod body plan consists of a body, head, and foot. A muscular bag called the mantle contains the cephalopod’s organs. The mantle has assumed many of the protective functions served by a shell in other mollusks. The cephalopod head contains the brain and sense organs. The foot, consisting of the grasping appendages, is fixed to the head, (hence “head-footed.”)

Appendages
Cephalopods are famous for the gangly limbs encircling their mouths. Octopuses, squid, and cuttlefish possess cone-shaped limbs studded with rows of suckers and referred to as arms. In addition, squid and cuttlefish brandish a pair of tentacles—long, elastic structures that dart out beyond the arms and are used exclusively for prey capture.

Three Hearts and Blue Blood
Cephalopods usually have three hearts. Two hearts pump blood to the gills, and one central heart pumps oxygenated blood to the body. Cephalopod blood is blue because it binds oxygen using a blue, copper-containing protein called hemocyanin. Human blood is red because the oxygen-binding protein hemoglobin contains iron.

Brain
Cephalopods have the largest brains of any invertebrate, and species of octopus, squid, and cuttlefish are capable of learning and retaining information.

  • Form Equals Function. Two large optic lobes in the squid brain testify to the importance of vision for locating prey. Likewise, octopuses, who rely on taste and texture to locate food, have highly developed lobes for storing chemical and tactile information.
  • Giant Axons. The nervous systems of squid are well known by scientists for their giant axons. The nerve cells of squid are the largest of any animal. With diameters up to one millimeter, they can be seen by the naked eye. Squid have been invaluable to neurological researchers who have used these oversized models to learn how human nerves function.

Eyes
Cephalopods have two eyes, which, in some species, are highly sophisticated structures similar to our own. Excluding the nautilus, the cephalopod eye contains a lens, retina, iris, and pupil. The resemblance to the human eye results from convergent evolution—human and cephalopod eyes, although similar in structure and function, arose independently, and not through a common ancestor. For another example of convergent evolution, compare porcupines with echidnas.

Ink Release
Most cephalopods are endowed with a small ink-producing gland within an ink sac, embedded in their digestive systems. When threatened by predators, the sac expels highly concentrated melanin to obscure the animal from predators. more

Skin
Cephalopod skin can take on wild patterns, colors, and shapes. Tens of thousands of organs called chromatophores form color patterns on the skin similar to the way pixels form images on a computer screen. The chromatophores are controlled by nerves, allowing color change to occur instantaneously. more

Jet propulsion
When they feel the need for speed, cephalopods propel themselves through the water using jet propulsion. Other methods of swimming involve fins or propulsion with the arms and web. more