Members of the class Cephalopoda include octopuses, squids, cuttlefishes, and chambered nautiluses. These marine mollusks are called cephalopods (SEF-uh-loh-PAHDZ), a term that means "head-foot." Cephalopods are specialized for a free-swimming, predatory existence. Extending from the head is a circle of tentacles, as you can see in Figure 35-8. The tentacles' powerful suction cups allow cephalopods to grasp objects and capture prey. Cephalopods kill and eat their prey with the help of a pair of jaws that resemble a parrot's beak.
The nervous system is more advanced in cephalopods than in any other group of mollusks. The cephalopod brain, which is the largest of any invertebrate brain, is divided into several lobes and contains millions of nerve cells. Octopuses, for example, can learn to perform tasks and discriminate between objects on the basis of their shape or texture. The sensory systems of cephalopods are also well developed. Most cephalopods have complex eyes that form images. The tentacles contain numerous cells that sense chemicals in the water.
Cephalopods have a closed circulatory system. Closed circulatory systems transport fluid more rapidly than open circulatory systems do. Thus, nutrients, oxygen, and carbon dioxide are carried quickly through the body of these highly active animals. Cephalopods also have separate sexes. The male uses a specialized tentacle to transfer packets of sperm from his mantle cavity to the mantle cavity of the female, where fertilization occurs. The female lays a mass of fertilized eggs and guards the eggs until they hatch. Unlike other mollusks, cephalopods develop from an egg into a juvenile without becoming a trochophore.
Many cephalopods can release a dark fluid into the water to temporarily distract predators. They also have pigment cells called chromatophores (kroh-MAT-uh-FAWHRZ), which are located in the outer layer of the mantle. Chromatophores can produce a sudden change in the color of a cephalopod, allowing the animal to blend in with its surroundings.
Squids are cephalopods with ten tentacles. The longest two tentacles are used for capturing prey, and the other eight tentacles force the prey into the squid's mouth. The muscular mantle propels the squid swiftly through the water by pumping jets of water through an excurrent siphon. Most squids grow to about 30 cm (1 ft) in length, but a few species can be much longer. The giant squid, Architeuthis, may reach a length of 18 m (about 60 ft) and a weight of more than 900 kg (about 1 ton). Architeuthis is the world's largest known invertebrate.
Octopuses have eight tentacles and share many characteristics with squids, including their methods of escaping from predators. Instead of using jet propulsion to chase prey, however, octopuses are more likely to crawl along the ocean bottom with their tentacles or lie in wait in caves and rock crevices. Octopuses average 1 m (3.3 ft) or less in length, although the giant Pacific octopus may grow to a length of 9 m (about 30 ft).
Squids and cuttlefish have small internal shells. The chambered nautilus, shown in Figure 35-9, is the only existing cephalopod that has retained its external shell. The nautilus shell is coiled and divided into a series of gas-filled chambers separated by partitions. The soft body of the nautilus is confined to the outermost chamber. As the nautilus grows, it moves forward in its shell, makes a new partition, and fills the chamber behind the partition with gas. The gas makes the nautilus buoyant.
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