Pääjalkaisiin

Pääjalkaisiin, scientific name Cephalopoda, is a class of marine molluscs. The name "cephalopod" literally means "head-foot" in Greek, referring to their characteristic arrangement of limbs extending from their head. This group includes well-known animals such as octopuses, squid, cuttlefish, and nautiluses.

Cephalopods are distinguished by their bilateral symmetry and a significant brain relative to their body size, making them the most intelligent of all invertebrates. They possess a specialized mantle, which encloses the visceral mass and often aids in locomotion through jet propulsion. Their arms or tentacles are equipped with suckers and are used for capturing prey, manipulating objects, and in some species, reproduction.

Many cephalopods have the ability to change their skin color and texture rapidly, a remarkable form of camouflage and communication. They typically have a complex nervous system, including eyes that are highly developed, comparable in complexity to vertebrate eyes.

The class Cephalopoda is divided into two subclasses. Coleoidea includes octopuses, squid, and cuttlefish, which generally have internal or reduced shells. Nautiloidea, represented by the nautilus, retains an external, chambered shell. Cephalopods are found in all of the world's oceans, from shallow coastal waters to the deep sea, and they play important roles in marine ecosystems as both predators and prey.