Cephalopods are a class of large, active animals with big heads and soft, fleshy bodies. They have long tentacles with suckers. They include the octopus, squid and nautilus. Only the nautilus lives in shells like most other mollusks. They have horny jaws and a tubular siphon that they use for both breathing and jet propulsion. They also have excellent sight. These animals can change color very quickly to match their emotions. This is done through cells that cause the blue, red, orange, yellow, violet or black granules in skin-pigment cells to spread rapidly and become more detectable. Some have light-producing organs that when stimulated will produce a bright glow by a chemical process. In some areas octopuses and squid are important food sources.
Octopuses live in holes, caves and even in empty tins and jars on the bottom of the sea, emerging to feed on small fish and crustaceans, which they kill by ejecting a poison through their beaks. They move slowly, pulling themselves along by the suckers on their eight tentacles. If threatened, octopuses can discharge a cloud of ink from a special sac that acts like a smoke screen, giving them time to escape their enemies by a form of locomotion like jet propulsion. The entrance to the mantle cavity is shut off and the muscle of the mantle contracted quickly, causing a jet of water to shoot from the open siphon and propel the animal rapidly backward.
This same siphon also draws water over the gills, thus giving the octopus its oxygen supply. An octopus leaves its shelter in search of food. Its main diet is mollusks. The giant octopus (Paroctopus defleini) lives in the North Pacific Ocean and is as much as 10 feet (3 m) across. It is not known to attack people, but it has a poisonous beak that should be avoided.
Squid and Cuttlefish
Squid has 10 tentacles, all with suction cups; eight are short, while two are long and slender and are used to seize prey. Squid can swim surprisingly fast, propelling their bodies by shooting jets of water through their siphons. Small lateral fins are used for moving more slowly through the water and helping keep the body stable. Squid eyes are almost as complex as those of humans.
Cuttlefish, a relative of the squid, can adjust the amount of gas and water in an internal skeleton so that they can move freely to higher or lower levels in the sea. When laying eggs the female squid grabs the string of eggs as they leave the siphon and attaches them to the ocean bottom or to floating weeds. The eggs hatch into free-swimming larvas. The largest of all invertebrates is the giant squid, a favorite food of sperm whales and probably the source of many sea monster stories.
Nautiluses are the only existing cephalopods that live in true shells. Although fossil remains show many different species, some with conical shells 15 feet (4.6 m) long, only three survive today. They have coiled shells similar to those of a snail, containing many chambers, but only the largest and most recently formed is inhabited. The others are filled with gas, which gives the shell buoyancy and keeps the animal afloat.
Nautiluses have many small, smooth tentacles, specialized for various Functions. Because they live in the depths by day and come up to within 100 feet (30m) of the surface only at night, they are rarely seen alive, though their shells are often washed ashore.
Echinoderms are simple animals lacking distinct heads and organs that are used specifically for respiration and excretion functions. The word Echinodermata comes from Greek, and it means spiny-skinned. This phylum includes starfish, sea cucumbers and sea urchins. Some of these, the starfish, for example, have arms or rays that are spaced evenly, and all have radially symmetrical bodies supported by pieces of crystalline calcium carbonate embedded in the skin.
Their bodies usually are arranged in parts of five around a central disk. The young are larval and are planktonic drifters, but the adults of most species live on the bottom. Though they are basically primitive, echinoderms do have some complicated organs. One of these is a system of water tubes running through the body. These tubes are connected to numerous smaller, cylindrical projections called tube feet, which are often provided with a sucker at their tip and are used for feeding and for moving around. They are also a simple means of respiration. Some echinoderms use their arms to move or to hold on to steep surfaces.
Starfish (Asteroidea) have five or more arms and a body with the mouth on the underside and the anus on the upper side. Their tube feet can be extended and used as suckers or withdraw. Contracting muscles can also block off the connections between different parts of the system of water tubes in their bodies. Starfish are carnivorous, some feeding on prey like mollusks by turning their stomachs inside out, engulfing the shellfish and pouring digestive juices on it. Respiration takes place through the tube feet and through apparatuses that protrude from gaps in the skeleton and function as gills. There are male and female starfish, but these cannot be distinguished by sight. Starfish also can regenerate an entire new animal from only one part of the body.
A crown-of-thorns starfish (Acanthaster planci) has dinner on a coral reef. This large starfish feeds on corals and causes great destruction of reefs. Its heavy spines are venomous and can cause great pain if they puncture the skin. It is often as much as 18 inches (46 cm) across and has 12 to 18 arms.
Brittle Stars and Basket Stars
The most abundant of all echinoderms are the brittle stars, or serpent stars (Ophiuroidea). One species found in England covers seabeds it lives on with as many as 100 million animals in a third of a square mile (0.85 km2). They have small, disk-shaped bodies with five arms and move the way snakes do.
Basket stars (Gorgonocephalidae) are unusual brittle stars, although they more closely resemble feather stars. They have multibranched, often tightly coiled arms. The name gorgonocephalid is derived from the basket star’s resemblance to the Gorgon, a monster in Greek mythology that had writhing snakes for hair.
The most primitive echinoderms, resembling flowers more than animals, are the feather stars, or sea lilies (Crinoidea). Long ago crinoids on stalks up to 60 feet (18m) long covered the sea bottom, but most of the present-day species are free-swimming creatures that lack stalks. Their long, jointed, flexible arms are branched, and they often swim by waving these arms up and down. Cirri, which are curved rootlike tentacles on the lower side, are used to hold on to seaweeds or rocks. The feather star catches small plankton and particles of food in streams of mucus that are carried along ciliated grooves in the arms to the mouth. Fertilization occurs in the sea, and the young go through a planktonic larval stage before they become adults.