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Sea Shell - Cephalopod

Sea Shell - Cephalopod - Scientific classification
Cephalopods make up the class Cephalopoda in the phylum Mollusca.

Sea Shell - Cephalopod - General Description
Cephalopod is the common name for any of the class of actively predatory marine mollusks, including the squid, octopus, and nautilus. The word cephalopod means, �head footed,� and the animals are so named because the arms surround the mouth.

Cephalopods are highly evolved animals in terms of structure and physiology, and the complexity of their behavior is equal to that of fish. Ecologically successful, they are among the more common predators in the sea; in turn many other animals, including humans, eat them. Giant squid, which can weigh as much as 900 kg (1980 lb), are the largest of all invertebrates. About 650 species of cephalopod are known.

Sea Shell - Cephalopod - Physical characteristics
The class is an ancient one, first appearing in the fossil record during the Cambrian period, about 600 million years ago. Primitive cephalopods, like other mollusks, had large external shells, but these were gradually reduced, as the animals grew faster and more active. The remaining primitive cephalopod, the nautilus, retains many archaic traits, such as an external shell with gas-filled chambers that aid flotation. The front of the nautiloid body protrudes from the opening of the shell and bears many sucker less arms. Below the head is a mantle cavity with four gills; a funnel around its opening ejects water to provide weak jet propulsion. The eyes lack lenses, and the nervous system is fairly simple. Prey is grasped with the tentacles and can be bitten with the mouth's sharp beak.

More advanced cephalopods are exemplified by the squid and cuttlefish, in which the shell is reduced and covered by tissue. The squid has a thin, horny, internal shell called a pen. The two-gilled mantle cavity is surrounded by muscles and provides much more effective jet propulsion, which is aided by fins. The squid has ten sucker-bearing arms, one pair of which is longer than the rest. The eyes are comparable in structure to the human eye, and the brain and nervous system are fairly complex. The octopus and its allies are even more modified. The shell is entirely absent, and the animal has only eight arms. Most octopuses are bottom dwellers, and a few attain a large size.

Digestion in cephalopods is rapid, and the circulatory and reproductive systems are well developed. The animals avoid predators mainly through flight or concealment, including an ability to change colors for camouflage, and some cephalopods eject a black secretion, called ink, to confuse predators. The sexes are separate; some species engage in complex mating displays. Cephalopod embryos develop in egg masses that are often cared for by the female.