Echinoderm, pronounced ih KY nuh durm, is the general name of certain spiny-skinned sea animals. There are about 6,000 kinds of echinoderms. Starfish, brittle stars, sand dollars, sea urchins, and sea cucumbers are among the most common kinds. All echinoderms have an internal bony skeleton. Their spines are a part of the skeleton. The echinoderm phylum (large animal group) is the only major phylum made up entirely of sea animals.
Adult echinoderms have radial symmetry. Their body parts are arranged around the center of the animal like the spokes of a wheel around the hub. Echinoderm bodies are usually divided into five sections with the mouth in the center.
Echinoderms are the only animals that have many tiny tubelike structures called tube feet. The tube feet project from the body in rows. Echinoderms use the tube feet for moving, feeding, breathing, and sensing. The outer tip of each tube often forms a suction disk for gripping hard surfaces. Within the echinoderm's body, a tiny bulb attached to the tube foot forces water into it to make it lengthen. An internal system of water-filled canals connects the tube feet to each other and to a sievelike plate that usually opens to the sea water. The entire system of tube feet and canals is called the water vascular system.
Echinoderms reproduce by laying eggs that develop into larvae and swim freely. The larvae have bilateral symmetry (two similar halves). The larvae sink to the ocean bottom and change into the adult, radial form.
Scientific classification. Echinoderms make up the echinoderm phylum, Echinodermata.
Wednesday, February 21, 2007
Mollusks
Mollusk, pronounced MAHL uhsk, is a soft-bodied animal that has no bones. Snails, slugs, clams, mussels, oysters, squids, and octopuses are mollusks. Most mollusks have a hard shell that protects their soft bodies. Some, such as cuttlefish and squids, have no outside shell. A special shell grows inside their bodies. This shell is called a cuttlebone in cuttlefish and a pen in squids. A few kinds of mollusks, including octopuses and certain slugs, have no shell at all. For additional information on mollusk shells and how they are formed.
All mollusks have a skinlike organ called the mantle. In mollusks with outside shells, the mantle makes the shell. The edges of the mantle release liquid shell materials and add them to the shell as the mollusk grows. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.
Mollusks live in most parts of the world. Some kinds of mollusks live in the deepest parts of oceans. Others are found on the wooded slopes of high mountains. Still others live in hot, dry deserts. Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.
Mollusks are used mainly for food. People in many parts of the world eat mollusks every day. Most Americans do not eat them nearly so often. The most popular kinds used as food in the United States are clams, oysters, and scallops. Mollusk shells are made into many useful products, including pearl buttons, jewelry, and various souvenir items. Perhaps the best-known mollusk products are the pearls made by pearl oysters.
Some mollusks are harmful to people. For example, certain small, freshwater snails of the tropics carry worms that cause an often fatal disease called schistosomiasis. Shipworm clams drill into rope, wooden boats, and wharves and cause millions of dollars worth of damage a year.
Mollusks make up the largest group of water animals. There are about 50,000 known kinds of living mollusks, and scientists find about 1,000 new species every year. The fossils of about 100,000 other species of mollusks have also been found.
The mollusks make up a phylum (major division) of the animal kingdom. The scientific name of the phylum is Mollusca, a Latin word meaning soft-bodied. To learn where the phylum fits into the animal kingdom, see ANIMAL.
There are seven classes (large groups) of mollusks. They are (1) univalves or Gastropoda, (2) bivalves or Bivalvia or Pelecypoda, (3) octopuses and squids or Cephalopoda, (4) tooth shells or Scaphopoda, (5) chitons or Polyplacophora, (6) Monoplacophora, and (7) Aplacophora.
Univalves or gastropods (Gastropoda) are the largest class of mollusks. They include limpets, slugs, snails, and whelks. Most kinds of univalves have a single, coiled shell. The name univalve comes from Latin words meaning one shell. But some kinds of univalves, including garden slugs and the sea slugs called nudibranchs, have no shells after the larval stage.
The name Gastropoda comes from Greek words meaning belly and foot. Gastropods seem to crawl on their bellies, but actually they use a large, muscular foot. The foot spreads beneath the body, and its muscles move in a rippling motion that makes the animal move forward. Most sea snails and some land snails have a lidlike part called an operculum on the back of the foot. When danger threatens, the snail draws back into its shell and the operculum closes the shell opening.
Certain kinds of univalves have two pairs of tentacles (feelers) on their heads. One pair helps the animals feel their way about. Some species have an eye on each of the other two tentacles. Others have no eyes at all. A univalve also has a ribbon of teeth. This ribbon, called a radula, works like a rough file and tears apart the animal's food. Most univalves that eat plants have thousands of weak teeth. A few kinds eat other mollusks, and have several dozen strong teeth.
Bivalves (Bivalvia or Pelecypoda) form the second largest class of mollusks. They include clams, oysters, mussels, scallops, and shipworms. All bivalves have two shells held together by hinges that look like small teeth. The shells of bivalves are usually open. When the animals are frightened, strong muscles pull the shells shut and hold them closed until danger has passed.
Bivalves have a strong, muscular foot. Many kinds of these animals move about by pushing the foot out and hooking it in the mud or sand. Then they pull themselves up with the foot. Some bivalves, such as the geoduck and razor clam, use the foot to dig holes. They push the foot downward into mud or sand. First the foot swells to enlarge the hole, and then it contracts and pulls the shell into the burrow. The Pholas clam can dig holes even in hard clay or soft rock.
Bivalves have no head or teeth. They get oxygen and food through a muscular siphon (tube). The siphon can be stretched to reach food and water if the animal is buried in mud or sand. Bivalves feed on plant cells material, which is filtered from the water by the gills.
Octopuses and squids (Cephalopoda) are the most active mollusks. The argonaut, cuttlefish, and nautilus also belong to this group. All the species in the group live in the ocean.
The word Cephalopoda comes from Greek words meaning head and foot. A cephalopod seems to be made up of a large head and long arms that look like feet. Octopuses and squids have dome-shaped "heads" surrounded by arms. Octopuses have eight arms, and squids have eight arms and two tentacles. The arms grow around hard, strong, beaklike jaws on the underside of the head. These jaws tear the animal's prey, and are far more dangerous than the arms. Octopuses use their arms and squids use their tentacles and arms to capture prey and pull it through their jaws. Octopuses and squids eat fish, other mollusks, and shellfish.
Tooth shells (Scaphopoda) have slender, curving shells that resemble tusks. These mollusks are often called tusk shells. The word Scaphopoda comes from Greek words that mean boat and foot. A tooth shell has a pointed foot that looks somewhat like a small boat. All tooth shells live in the ocean, where they burrow in the mud or sand. The top of the shell sticks up into the water. Tooth shells have no head or eyes. They feed on one-celled organisms that are swept into the mouth by tentacles.
Chitons (Polyplacophora) have flat, oval bodies covered by eight shell plates. The plates are held together by a tough girdle. The name Polyplacophora comes from Greek words that mean many, shell, and bearer. This name refers to the eight overlapping pieces of a chiton's shell. Chitons have a large, flat foot. They can use the foot to move about, but they usually cling firmly to rocks. When they are forced to let go of the rocks, they roll up into a ball. Chitons have a small head and mouth, but they have no eyes or tentacles. Their long radula has many teeth, which some chitons use to scrape seaweed from rocks for food.
Monoplacophora live in the deep parts of the ocean, and most are found only as fossils. The name Monoplacophora comes from Greek words meaning single, shell, and bearer. Monoplacophorans have one shell that is almost flat, like a limpet shell. They are unusual because they have several pairs of gills, six or more pairs of kidneys, and many ladderlike nerve centers. Like other mollusks, they have a mantle. They also have a radula. Little is known about their habits.
Aplacophora are rarely seen, wormlike mollusks covered with small spines. The name Aplacophora comes from Greek words that mean no shell.
All mollusks have a skinlike organ called the mantle. In mollusks with outside shells, the mantle makes the shell. The edges of the mantle release liquid shell materials and add them to the shell as the mollusk grows. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.
Mollusks live in most parts of the world. Some kinds of mollusks live in the deepest parts of oceans. Others are found on the wooded slopes of high mountains. Still others live in hot, dry deserts. Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.
The importance of mollusks
Mollusks are used mainly for food. People in many parts of the world eat mollusks every day. Most Americans do not eat them nearly so often. The most popular kinds used as food in the United States are clams, oysters, and scallops. Mollusk shells are made into many useful products, including pearl buttons, jewelry, and various souvenir items. Perhaps the best-known mollusk products are the pearls made by pearl oysters.
Some mollusks are harmful to people. For example, certain small, freshwater snails of the tropics carry worms that cause an often fatal disease called schistosomiasis. Shipworm clams drill into rope, wooden boats, and wharves and cause millions of dollars worth of damage a year.
Kinds of mollusks
Mollusks make up the largest group of water animals. There are about 50,000 known kinds of living mollusks, and scientists find about 1,000 new species every year. The fossils of about 100,000 other species of mollusks have also been found.
The mollusks make up a phylum (major division) of the animal kingdom. The scientific name of the phylum is Mollusca, a Latin word meaning soft-bodied. To learn where the phylum fits into the animal kingdom, see ANIMAL.
There are seven classes (large groups) of mollusks. They are (1) univalves or Gastropoda, (2) bivalves or Bivalvia or Pelecypoda, (3) octopuses and squids or Cephalopoda, (4) tooth shells or Scaphopoda, (5) chitons or Polyplacophora, (6) Monoplacophora, and (7) Aplacophora.
Univalves or gastropods (Gastropoda) are the largest class of mollusks. They include limpets, slugs, snails, and whelks. Most kinds of univalves have a single, coiled shell. The name univalve comes from Latin words meaning one shell. But some kinds of univalves, including garden slugs and the sea slugs called nudibranchs, have no shells after the larval stage.
The name Gastropoda comes from Greek words meaning belly and foot. Gastropods seem to crawl on their bellies, but actually they use a large, muscular foot. The foot spreads beneath the body, and its muscles move in a rippling motion that makes the animal move forward. Most sea snails and some land snails have a lidlike part called an operculum on the back of the foot. When danger threatens, the snail draws back into its shell and the operculum closes the shell opening.
Certain kinds of univalves have two pairs of tentacles (feelers) on their heads. One pair helps the animals feel their way about. Some species have an eye on each of the other two tentacles. Others have no eyes at all. A univalve also has a ribbon of teeth. This ribbon, called a radula, works like a rough file and tears apart the animal's food. Most univalves that eat plants have thousands of weak teeth. A few kinds eat other mollusks, and have several dozen strong teeth.
Bivalves (Bivalvia or Pelecypoda) form the second largest class of mollusks. They include clams, oysters, mussels, scallops, and shipworms. All bivalves have two shells held together by hinges that look like small teeth. The shells of bivalves are usually open. When the animals are frightened, strong muscles pull the shells shut and hold them closed until danger has passed.
Bivalves have a strong, muscular foot. Many kinds of these animals move about by pushing the foot out and hooking it in the mud or sand. Then they pull themselves up with the foot. Some bivalves, such as the geoduck and razor clam, use the foot to dig holes. They push the foot downward into mud or sand. First the foot swells to enlarge the hole, and then it contracts and pulls the shell into the burrow. The Pholas clam can dig holes even in hard clay or soft rock.
Bivalves have no head or teeth. They get oxygen and food through a muscular siphon (tube). The siphon can be stretched to reach food and water if the animal is buried in mud or sand. Bivalves feed on plant cells material, which is filtered from the water by the gills.
Octopuses and squids (Cephalopoda) are the most active mollusks. The argonaut, cuttlefish, and nautilus also belong to this group. All the species in the group live in the ocean.
The word Cephalopoda comes from Greek words meaning head and foot. A cephalopod seems to be made up of a large head and long arms that look like feet. Octopuses and squids have dome-shaped "heads" surrounded by arms. Octopuses have eight arms, and squids have eight arms and two tentacles. The arms grow around hard, strong, beaklike jaws on the underside of the head. These jaws tear the animal's prey, and are far more dangerous than the arms. Octopuses use their arms and squids use their tentacles and arms to capture prey and pull it through their jaws. Octopuses and squids eat fish, other mollusks, and shellfish.
Tooth shells (Scaphopoda) have slender, curving shells that resemble tusks. These mollusks are often called tusk shells. The word Scaphopoda comes from Greek words that mean boat and foot. A tooth shell has a pointed foot that looks somewhat like a small boat. All tooth shells live in the ocean, where they burrow in the mud or sand. The top of the shell sticks up into the water. Tooth shells have no head or eyes. They feed on one-celled organisms that are swept into the mouth by tentacles.
Chitons (Polyplacophora) have flat, oval bodies covered by eight shell plates. The plates are held together by a tough girdle. The name Polyplacophora comes from Greek words that mean many, shell, and bearer. This name refers to the eight overlapping pieces of a chiton's shell. Chitons have a large, flat foot. They can use the foot to move about, but they usually cling firmly to rocks. When they are forced to let go of the rocks, they roll up into a ball. Chitons have a small head and mouth, but they have no eyes or tentacles. Their long radula has many teeth, which some chitons use to scrape seaweed from rocks for food.
Monoplacophora live in the deep parts of the ocean, and most are found only as fossils. The name Monoplacophora comes from Greek words meaning single, shell, and bearer. Monoplacophorans have one shell that is almost flat, like a limpet shell. They are unusual because they have several pairs of gills, six or more pairs of kidneys, and many ladderlike nerve centers. Like other mollusks, they have a mantle. They also have a radula. Little is known about their habits.
Aplacophora are rarely seen, wormlike mollusks covered with small spines. The name Aplacophora comes from Greek words that mean no shell.
Arthropods
Arthropod, pronounced AHR thruh pahd, is any animal that belongs to the major division, or phylum, of the animal kingdom called the Arthropoda. This term is formed from two Greek words, and means jointed feet. Actually, the legs, rather than the feet, are jointed. All the Arthropoda, or arthropods, have jointed legs. Among the most important groups of arthropods are the following: (1) insects, including cockroaches, beetles, bees, butterflies, and many others; (2) crustaceans, including such well-known animals as crabs, lobsters, shrimps, and barnacles; (3) arachnids, including mites, ticks, spiders, and scorpions; (4) chilopods, or centipedes; and (5) diplo-pods, or millipedes. The arthropod phylum contains more than three-fourths of all the different kinds of animals. Insects make up the largest class of arthropods in terms of the number of species.
The bodies of arthropods, as well as their legs, are made up of sections. Among some primitive arthropods, each section of the body has its own pair of legs. Most of these legs are used for swimming or walking. In some types of arthropods, certain legs have developed special shapes and uses. Some serve as sucking organs, some are jaws, some serve as weapons of offense and defense, and some are sense organs. Insects lack most of the pairs of legs found in other arthropods. They have only three pair. One pair is attached to each segment of an insect's chest or thorax. Insects also may have one or two pairs of wings.
Arthropods have an outside shell, or exoskeleton, that contains a stiff, horny material called chitin. Certain arthropods, such as flies and moths, have only thin, weak shells. Others, including crabs and lobsters, have thick, strong shells. Nearly all arthropods have a kind of heart and blood system and usually a well-organized nervous system. Some arthropods have simple eyes, some have compound eyes, and some (including many insects) have eyes of both types.
The bodies of arthropods, as well as their legs, are made up of sections. Among some primitive arthropods, each section of the body has its own pair of legs. Most of these legs are used for swimming or walking. In some types of arthropods, certain legs have developed special shapes and uses. Some serve as sucking organs, some are jaws, some serve as weapons of offense and defense, and some are sense organs. Insects lack most of the pairs of legs found in other arthropods. They have only three pair. One pair is attached to each segment of an insect's chest or thorax. Insects also may have one or two pairs of wings.
Arthropods have an outside shell, or exoskeleton, that contains a stiff, horny material called chitin. Certain arthropods, such as flies and moths, have only thin, weak shells. Others, including crabs and lobsters, have thick, strong shells. Nearly all arthropods have a kind of heart and blood system and usually a well-organized nervous system. Some arthropods have simple eyes, some have compound eyes, and some (including many insects) have eyes of both types.
Nematodes
Roundworm, also called nematode or eelworm, is any of more than 12,000 species of worms. Many species of roundworms live freely in soil, water, dead plants, or dead animals. All other roundworms are parasites. They live and feed on living plants and animals, which serve as hosts. Some parasitic species cause serious diseases in human beings and other hosts.
Roundworms range in size from microscopic to more than 3 feet (90 centimeters) long. They have slender, round bodies with tapered ends. Roundworms have remarkable powers of reproduction and are extremely numerous. Researchers have found more than 90,000 roundworms in a single rotting apple.
Nearly all species of roundworms reproduce by laying eggs. Some species produce great quantities of eggs. For example, females of the species Ascaris lumbricoides each lay about 200,000 eggs per day for at least 10 months. Among some species of roundworms, the eggs hatch into tiny young that look like the adults. Eggs of other species hatch into young called larvae, which gradually transform into adults. Species of roundworms that do not lay eggs give birth to larvae.
Parasitic roundworms may infect a host in a number of ways. Some species enter the host when the host swallows food that contains the roundworm, its eggs, or its larvae. Among other species, the larva burrows into the skin of the host. In other species of roundworms, the larva is taken up by an insect, such as a fly or a mosquito, and transmitted through the bite of that insect to the host.
At least 14 species of roundworms cause infection in human beings. A. lumbricoides, which inhabits the small intestine, infects about 65 million people throughout the world. It causes a disease called ascariasis. Symptoms of this disease include pneumonia and intestinal pain. The roundworm Trichuris trichiura infects the large intestine and occurs in about 350 million people worldwide. It causes trichuriasis, a disease characterized by diarrhea. Other common roundworms that cause disease in humans include filariae, hookworms, pinworms, and trichinae.
Roundworms range in size from microscopic to more than 3 feet (90 centimeters) long. They have slender, round bodies with tapered ends. Roundworms have remarkable powers of reproduction and are extremely numerous. Researchers have found more than 90,000 roundworms in a single rotting apple.
Nearly all species of roundworms reproduce by laying eggs. Some species produce great quantities of eggs. For example, females of the species Ascaris lumbricoides each lay about 200,000 eggs per day for at least 10 months. Among some species of roundworms, the eggs hatch into tiny young that look like the adults. Eggs of other species hatch into young called larvae, which gradually transform into adults. Species of roundworms that do not lay eggs give birth to larvae.
Parasitic roundworms may infect a host in a number of ways. Some species enter the host when the host swallows food that contains the roundworm, its eggs, or its larvae. Among other species, the larva burrows into the skin of the host. In other species of roundworms, the larva is taken up by an insect, such as a fly or a mosquito, and transmitted through the bite of that insect to the host.
At least 14 species of roundworms cause infection in human beings. A. lumbricoides, which inhabits the small intestine, infects about 65 million people throughout the world. It causes a disease called ascariasis. Symptoms of this disease include pneumonia and intestinal pain. The roundworm Trichuris trichiura infects the large intestine and occurs in about 350 million people worldwide. It causes trichuriasis, a disease characterized by diarrhea. Other common roundworms that cause disease in humans include filariae, hookworms, pinworms, and trichinae.
Rotifers
Rotifer, pronounced ROH tuh fuhr, is a type of tiny multicellular animal that lives in water. The largest rotifers are about 1/26 inch (1 millimeter) long. Rotifers have cylinder- or vase-shaped bodies. Most species live in lakes, rivers, or streams. Some live in the ocean.
The name rotifer means wheel bearer and refers to the circles of hairlike projections called cilia on the animal's head. The cilia create a circular water current that draws food to the rotifer. This water current also enables most species of rotifers to "swim." Other species spend their entire lives attached to such objects as stones and leaves.
In many rotifer populations, the male has no role in reproduction. The female produces young by herself. This kind of reproduction, called parthenogenesis, produces only female offspring.
Scientific classification. Rotifers make up the phylum Rotifera.
The name rotifer means wheel bearer and refers to the circles of hairlike projections called cilia on the animal's head. The cilia create a circular water current that draws food to the rotifer. This water current also enables most species of rotifers to "swim." Other species spend their entire lives attached to such objects as stones and leaves.
In many rotifer populations, the male has no role in reproduction. The female produces young by herself. This kind of reproduction, called parthenogenesis, produces only female offspring.
Scientific classification. Rotifers make up the phylum Rotifera.
Platyhelminthes
Flatworm is a kind of worm. Some flatworms live freely on land or in water. Others live as parasites in human beings or other animals. Many flatworms, especially the larger species, have a flat body.
Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal's body. An inner layer of cells forms an intestine in most flatworms. A tightly packed mass of cells called the parenchyma fills the body between the epidermis and intestine. Muscles, glands, nerves, and reproductive organs lie in the parenchyma. The only opening of the intestine is the animal's mouth. The mouth may be at the head end, the rear end, or the underside of the body.
Many flatworms have a smooth, soft body. Many have suckers or other projections on the body. Some flatworms have spines and tiny, needlelike spicules that serve as a kind of skeleton. Most flatworms measure less than 1 inch (2.5 centimeters) long. However, the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.
There are about 13,000 species of flatworms. They may be divided into four groups: (1) turbellarians; (2) monogeneans; (3) trematodes; and (4) cestodes, or tapeworms. Most turbellarians are free-living. They usually are found in sand and mud on the bottom of bodies of water. A few species live on land in moist soil. The other three groups of flatworms are parasites. They live in a wide variety of organisms that serve as hosts.
Almost all flatworms are hermaphroditic--that is, both male and female reproductive organs are found in the same animal. Most turbellarians lay eggs that hatch into tiny young that resemble the adults. In some turbellarians, and in all parasitic flatworms, young--called larvae--hatch from the eggs. The larvae look different from the adults and live in different habitats. For example, the larva of a monogenean has hairlike cilia that enable it to swim. The larva swims until it finds an appropriate fish for a host. The larva attaches to the fish and develops into an adult. The adult, which lives on the skin and gills of the fish, lacks cilia and cannot swim.
Parasitic flatworms cause disease in their hosts. Schistosomiasis, for example, is a tropical disease caused by schistosomes (blood flukes) living in the blood vessels of the abdomen. Adult tapeworms that live in the intestine of human beings do not usually cause much harm. However, tapeworm larvae cause serious diseases that can be fatal if not treated.
Scientific classification. Flatworms make up the phylum Platyhelminthes. The four classes of flatworms are Turbellaria, Monogenea, Trematoda, and Cestoda.
Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal's body. An inner layer of cells forms an intestine in most flatworms. A tightly packed mass of cells called the parenchyma fills the body between the epidermis and intestine. Muscles, glands, nerves, and reproductive organs lie in the parenchyma. The only opening of the intestine is the animal's mouth. The mouth may be at the head end, the rear end, or the underside of the body.
Many flatworms have a smooth, soft body. Many have suckers or other projections on the body. Some flatworms have spines and tiny, needlelike spicules that serve as a kind of skeleton. Most flatworms measure less than 1 inch (2.5 centimeters) long. However, the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.
There are about 13,000 species of flatworms. They may be divided into four groups: (1) turbellarians; (2) monogeneans; (3) trematodes; and (4) cestodes, or tapeworms. Most turbellarians are free-living. They usually are found in sand and mud on the bottom of bodies of water. A few species live on land in moist soil. The other three groups of flatworms are parasites. They live in a wide variety of organisms that serve as hosts.
Almost all flatworms are hermaphroditic--that is, both male and female reproductive organs are found in the same animal. Most turbellarians lay eggs that hatch into tiny young that resemble the adults. In some turbellarians, and in all parasitic flatworms, young--called larvae--hatch from the eggs. The larvae look different from the adults and live in different habitats. For example, the larva of a monogenean has hairlike cilia that enable it to swim. The larva swims until it finds an appropriate fish for a host. The larva attaches to the fish and develops into an adult. The adult, which lives on the skin and gills of the fish, lacks cilia and cannot swim.
Parasitic flatworms cause disease in their hosts. Schistosomiasis, for example, is a tropical disease caused by schistosomes (blood flukes) living in the blood vessels of the abdomen. Adult tapeworms that live in the intestine of human beings do not usually cause much harm. However, tapeworm larvae cause serious diseases that can be fatal if not treated.
Scientific classification. Flatworms make up the phylum Platyhelminthes. The four classes of flatworms are Turbellaria, Monogenea, Trematoda, and Cestoda.
Annelids
Segmented worms are the most highly developed worms. Their body consists of segments that give the worms a ringed appearance. This group includes polychaete worms, oligochaete worms, and leeches.
Polychaete worms, the largest group of segmented worms, live in the sea and along the shore. Many of these worms have tentacles (feelers) on their head and a pair of leglike projections called parapodia on each body segment. The parapodia are used in crawling. They have many setae (bristles) that help the worms grip the surface on which they are moving. Many polychaete worms live among algae or burrow in mud or sand. Some live in tubes attached to the sea floor. A worm makes its tube from sand or from material secreted by its body. Some polychaete worms eat small plants and animals. Others feed on plant and animal remains.
Oligochaete worms include earthworms and many freshwater species. They have a few setae but no parapodia. Most oligochaete worms eat decaying plant matter.
Leeches make up the smallest group of segmented worms. They grow from 3/8 to 12 inches (1 to 30 centimeters) long and have a flat body with a sucker at each end. Most leeches live in water and feed on the blood of fish and other water creatures.
Polychaete worms, the largest group of segmented worms, live in the sea and along the shore. Many of these worms have tentacles (feelers) on their head and a pair of leglike projections called parapodia on each body segment. The parapodia are used in crawling. They have many setae (bristles) that help the worms grip the surface on which they are moving. Many polychaete worms live among algae or burrow in mud or sand. Some live in tubes attached to the sea floor. A worm makes its tube from sand or from material secreted by its body. Some polychaete worms eat small plants and animals. Others feed on plant and animal remains.
Oligochaete worms include earthworms and many freshwater species. They have a few setae but no parapodia. Most oligochaete worms eat decaying plant matter.
Leeches make up the smallest group of segmented worms. They grow from 3/8 to 12 inches (1 to 30 centimeters) long and have a flat body with a sucker at each end. Most leeches live in water and feed on the blood of fish and other water creatures.
Cnidarians
Cnidarian, pronounced ny DAIR ee uhn, is the name of a group of soft-bodied water animals. The group includes the freshwater hydras, hydroids, jellyfish, sea fans, sea anemones, and corals. These animals make up the phylum (large group) called Cnidaria. There are approximately 9,000 species of cnidarians, and most of them live in the sea. Cnidarians are also called coelenterates.
The body of a cnidarian may be shaped like a cylinder, a bell, or an umbrella. The mouth opens at one end and leads to a digestive cavity. Every cnidarian has at least two layers of cells that form its body wall. An outer layer makes up the body covering, and an inner layer lines the digestive cavity. Many cnidarians have a third, or middle, layer consisting of a stiff, jellylike material that helps support the animal.
A medusa, or jellyfish, is a cnidarian that has a bell- or umbrella-shaped body. Its mouth is at the underside of the body. Tentacles with special stinging cells hang downward from the body's ringlike edge. Medusas swim about freely in the sea.
A polyp is a cnidarian that has a body shaped like a hollow cylinder. A polyp lives with one end of its body attached to the sea bottom. The mouth and tentacles extend upward at the other end. Polyps may exist singly or may live together in colonies. Colonies are produced when polyps form buds that detach and become new polyps. Hydras and sea anemones are examples of single polyps, and hydroids and most corals are colony-forming polyps.
Some cnidarians have either medusa or polyp stages, or both, in their life cycles. The medusas are produced from special polyp buds that eventually break free and swim away. Then the medusas produce eggs and sperm that unite and develop into polyps.
The body of a cnidarian may be shaped like a cylinder, a bell, or an umbrella. The mouth opens at one end and leads to a digestive cavity. Every cnidarian has at least two layers of cells that form its body wall. An outer layer makes up the body covering, and an inner layer lines the digestive cavity. Many cnidarians have a third, or middle, layer consisting of a stiff, jellylike material that helps support the animal.
A medusa, or jellyfish, is a cnidarian that has a bell- or umbrella-shaped body. Its mouth is at the underside of the body. Tentacles with special stinging cells hang downward from the body's ringlike edge. Medusas swim about freely in the sea.
A polyp is a cnidarian that has a body shaped like a hollow cylinder. A polyp lives with one end of its body attached to the sea bottom. The mouth and tentacles extend upward at the other end. Polyps may exist singly or may live together in colonies. Colonies are produced when polyps form buds that detach and become new polyps. Hydras and sea anemones are examples of single polyps, and hydroids and most corals are colony-forming polyps.
Some cnidarians have either medusa or polyp stages, or both, in their life cycles. The medusas are produced from special polyp buds that eventually break free and swim away. Then the medusas produce eggs and sperm that unite and develop into polyps.
Porifera
Sponge is an animal that lives at the bottom of oceans and other bodies of water. Sponges do not have heads, arms, or internal organs. They live attached to rocks, plants, and other objects beneath the water's surface. Adult sponges do not move about from one place to another, and many sponges look like plants. For these reasons, people once regarded sponges as plants. But today, scientists classify sponges as animals. Like most animals, sponges eat their food. They cannot manufacture their own food, as do plants.
There are about 5,000 species of sponges. Most of them live in oceans, but a few species are found in lakes, rivers, and other bodies of fresh water. Sponges can live in both shallow and deep water. Most marine (ocean-dwelling) sponges inhabit warm or tropical seas.
Sponges are among the oldest kinds of animals. Fossils have been found of marine sponges that lived more than 500 million years ago. For centuries, people have used sponges for cleaning and bathing. The skeletons of certain sponges make good cleaning tools because they are soft and absorb large amounts of water. Commercial fishing crews still harvest bath sponges in the Gulf Stream and the Mediterranean Sea. However, most cleaning sponges are artificially produced.
Recently, scientists have discovered chemical compounds in sponges that may be used in medicines to fight cancer and other diseases. Because sponges harbor large populations of bacteria in their body tissues, the bacteria may produce many of these compounds. Such important discoveries have led to an increased amount of research involving sponges.
Sponges vary widely in shape, color, and size. Some sponges are round while others are shaped like vases. Many simply follow the shape of the object on which they grow, forming a living crust. Marine sponges range in color from bright yellow, orange, or purple, to gray or brown. Sponges of the same species may be of many different colors. Most freshwater sponges are green, purple, or gray. The smallest sponges measure less than 1 inch (2.5 centimeters) in diameter. The largest grow to more than 4 feet (1.2 meters) in diameter.
Body openings. A sponge has two types of openings on its body surface: (1) small pores called ostia, and (2) a large osculum. The sponge's ostia allow water to enter its body, and the osculum allows water to leave the body. Among more advanced sponges, a network of canals transports water entering through the ostia to all parts of the sponge. The water brings tiny plants and animals into the sponge. These tiny organisms are the sponge's food. Waste products--along with water--leave the sponge through the osculum.
Special cells. The canals that pass into the sponge's body lead into many small chambers. These small chambers in the sponge are lined with cells called choanocytes, also known as collar cells. Each of these cells has a delicate tissue, or collar, that acts like a net to trap food particles. Each collar cell also bears a long threadlike structure called a flagellum. The sponge's flagella whip around, and this action creates the water currents that flow through the body of the sponge.
In addition to collar cells, the sponge's body also contains other types of cells. Some of these cells form tissue that covers the sponge's body and the walls of the canals inside the body. Other types of cells travel freely within the sponge. These cells have many different functions. For example, some heal injuries to the body and others play a major role in reproduction. Still others produce material for the sponge's skeleton.
Skeleton. Sponges have several types of skeletons. Most sponges have a mineral skeleton made up of tiny, needlelike spicules. The spicules may be of either calcium carbonate (limestone) or silica, a glasslike mineral. In bath sponges, the skeleton consists only of fibers of a tan-colored protein called spongin. The skeleton of spongin fibers is what remains after a bath sponge dies and its cells are removed. Many sponges have a skeleton of both mineral spicules and spongin fibers. In other sponges, the skeleton consists of silica spicules, spongin fibers, and a massive base of limestone crystals.
The sponge's skeleton forms a framework that supports and protects the body. Spicules may be organized into bundles that form strong, geometric networks. In many sponges, numerous spicules grow around the osculum. These spicules protect the sponge from animals that try to eat it or enter its body.
Sponges reproduce both sexually and asexually. In sexual reproduction, a new sponge develops from the joining of two sex cells. In asexual reproduction, a new sponge is formed by methods that do not involve sex cells. Most sponges also have the ability to replace lost or injured body parts by growing new ones. This process is called regeneration.
Sexual reproduction in sponges begins when an egg (female sex cell) starts to grow inside the parent sponge's body. At first, the egg absorbs food from surrounding body fluids. Later, it engulfs cells called nurse cells, which provide food reserves. When fully grown, the egg is fertilized by a sperm (male sex cell). Some sponges produce both eggs and sperm. In these species, the egg may be fertilized by a sperm from the same animal.
Other species produce either eggs or sperm only. In these species, another sponge releases sperm into the surrounding water. A sperm enters the parent sponge's body by way of the ostia and canal network and fertilizes the egg.
After the egg is fertilized, it gradually develops into a larva (immature animal form). The larva is covered with cells that have flagella. The flagella beat rapidly, enabling the larva to swim outward through the parent's canal system, aided by water currents. The larva leaves the sponge through the osculum and swims around from a few hours to a few days. It then attaches itself to some suitable surface at the bottom of the body of water and develops into an adult sponge.
Asexual reproduction in sponges may occur in a variety of ways. In every case, however, it involves cells called archaeocytes. These cells have no specialized functions. Instead, they have the capacity to develop into any type of cell in the sponge's body. During asexual reproduction, a group of archaeocytes grow into every type of cell needed to form a new sponge.
Sponges may reproduce asexually by budding. In this process, buds or branches filled with archaeocytes grow on the parent sponge. These growths may break away from or fall off the parent sponge or remain attached to it. The growths eventually develop into new sponges.
Some marine sponges and most freshwater sponges also may reproduce asexually by forming gemmules. Gemmules are budlike structures that consist of a group of archaeocytes within a tough shell of spongin. Many gemmules are reinforced by spicules. Gemmules typically form in response to either cold or hot weather. Protected within the gemmule shell, the archaeocytes can survive periods of drought or freezing temperatures, though the parent sponge may die. Gemmules "hatch" when more favorable weather returns. The archaeocytes then spread out on a solid surface and develop into a new sponge.
Regeneration. The developmental abilities of archaeocytes give sponges remarkable powers of regeneration. Even if large parts of a sponge's body are lost or damaged, they may be replaced or repaired. In laboratory experiments, scientists have pressed sponges through extremely fine cloth so that the bodies of the sponges break up into separate cells or clumps of cells. When these cells are replaced in water, they first migrate together to form rounded cell clusters. Then the cell clusters reorganize to form complete sponges again.
Sponges make up a phylum (major group) of animals called Porifera, which comes from a Latin word meaning pore-bearer. Zoologists divide sponges into three classes, based chiefly on common skeletal features.
Sponges with a limestone skeleton belong to the class Calcarea. Most species in this class inhabit shallow parts of oceans, but some have been found at depths of up to 13,000 feet (4,000 meters). The tiny sponge called Sycon belongs to this group.
A second class, called Hexactinellida, consists of marine sponges with a silica skeleton. These species are commonly called glass sponges. Their spicules form beautiful geometric patterns. Glass sponges live as deep as 23,000 feet (7,000 meters) beneath the ocean's surface. The Venus's-flower-basket is a typical kind of glass sponge.
All freshwater sponges and most of the best-known marine sponges are in the class Demospongiae. Most of these animals have a skeleton of silica or spongin or of both substances. One kind of sponge in this class, the boring sponge, bores into coral, seashells, and other hard structures. This activity helps shape such marine environments as coral reefs and seacoasts. Other marine species in this group of sponges include the red-beard sponge, the sheepswool sponge, and bath sponges.
Some ocean sponges have a skeleton of silica and spongin with a thick base of limestone. Scientists include these sponges, sometimes called coralline sponges, in either the class Calcarea or Demospongiae. Many coralline sponges live in underwater caves. They are closely related to marine sponges that lived hundreds of millions of years ago.
Scientific classification. Sponges make up the phylum Porifera, which is divided into the classes Calcarea, Demospongiae, and Hexactinellida.
There are about 5,000 species of sponges. Most of them live in oceans, but a few species are found in lakes, rivers, and other bodies of fresh water. Sponges can live in both shallow and deep water. Most marine (ocean-dwelling) sponges inhabit warm or tropical seas.
Sponges are among the oldest kinds of animals. Fossils have been found of marine sponges that lived more than 500 million years ago. For centuries, people have used sponges for cleaning and bathing. The skeletons of certain sponges make good cleaning tools because they are soft and absorb large amounts of water. Commercial fishing crews still harvest bath sponges in the Gulf Stream and the Mediterranean Sea. However, most cleaning sponges are artificially produced.
Recently, scientists have discovered chemical compounds in sponges that may be used in medicines to fight cancer and other diseases. Because sponges harbor large populations of bacteria in their body tissues, the bacteria may produce many of these compounds. Such important discoveries have led to an increased amount of research involving sponges.
The bodies of sponges
Sponges vary widely in shape, color, and size. Some sponges are round while others are shaped like vases. Many simply follow the shape of the object on which they grow, forming a living crust. Marine sponges range in color from bright yellow, orange, or purple, to gray or brown. Sponges of the same species may be of many different colors. Most freshwater sponges are green, purple, or gray. The smallest sponges measure less than 1 inch (2.5 centimeters) in diameter. The largest grow to more than 4 feet (1.2 meters) in diameter.
Body openings. A sponge has two types of openings on its body surface: (1) small pores called ostia, and (2) a large osculum. The sponge's ostia allow water to enter its body, and the osculum allows water to leave the body. Among more advanced sponges, a network of canals transports water entering through the ostia to all parts of the sponge. The water brings tiny plants and animals into the sponge. These tiny organisms are the sponge's food. Waste products--along with water--leave the sponge through the osculum.
Special cells. The canals that pass into the sponge's body lead into many small chambers. These small chambers in the sponge are lined with cells called choanocytes, also known as collar cells. Each of these cells has a delicate tissue, or collar, that acts like a net to trap food particles. Each collar cell also bears a long threadlike structure called a flagellum. The sponge's flagella whip around, and this action creates the water currents that flow through the body of the sponge.
In addition to collar cells, the sponge's body also contains other types of cells. Some of these cells form tissue that covers the sponge's body and the walls of the canals inside the body. Other types of cells travel freely within the sponge. These cells have many different functions. For example, some heal injuries to the body and others play a major role in reproduction. Still others produce material for the sponge's skeleton.
Skeleton. Sponges have several types of skeletons. Most sponges have a mineral skeleton made up of tiny, needlelike spicules. The spicules may be of either calcium carbonate (limestone) or silica, a glasslike mineral. In bath sponges, the skeleton consists only of fibers of a tan-colored protein called spongin. The skeleton of spongin fibers is what remains after a bath sponge dies and its cells are removed. Many sponges have a skeleton of both mineral spicules and spongin fibers. In other sponges, the skeleton consists of silica spicules, spongin fibers, and a massive base of limestone crystals.
The sponge's skeleton forms a framework that supports and protects the body. Spicules may be organized into bundles that form strong, geometric networks. In many sponges, numerous spicules grow around the osculum. These spicules protect the sponge from animals that try to eat it or enter its body.
How sponges reproduce
Sponges reproduce both sexually and asexually. In sexual reproduction, a new sponge develops from the joining of two sex cells. In asexual reproduction, a new sponge is formed by methods that do not involve sex cells. Most sponges also have the ability to replace lost or injured body parts by growing new ones. This process is called regeneration.
Sexual reproduction in sponges begins when an egg (female sex cell) starts to grow inside the parent sponge's body. At first, the egg absorbs food from surrounding body fluids. Later, it engulfs cells called nurse cells, which provide food reserves. When fully grown, the egg is fertilized by a sperm (male sex cell). Some sponges produce both eggs and sperm. In these species, the egg may be fertilized by a sperm from the same animal.
Other species produce either eggs or sperm only. In these species, another sponge releases sperm into the surrounding water. A sperm enters the parent sponge's body by way of the ostia and canal network and fertilizes the egg.
After the egg is fertilized, it gradually develops into a larva (immature animal form). The larva is covered with cells that have flagella. The flagella beat rapidly, enabling the larva to swim outward through the parent's canal system, aided by water currents. The larva leaves the sponge through the osculum and swims around from a few hours to a few days. It then attaches itself to some suitable surface at the bottom of the body of water and develops into an adult sponge.
Asexual reproduction in sponges may occur in a variety of ways. In every case, however, it involves cells called archaeocytes. These cells have no specialized functions. Instead, they have the capacity to develop into any type of cell in the sponge's body. During asexual reproduction, a group of archaeocytes grow into every type of cell needed to form a new sponge.
Sponges may reproduce asexually by budding. In this process, buds or branches filled with archaeocytes grow on the parent sponge. These growths may break away from or fall off the parent sponge or remain attached to it. The growths eventually develop into new sponges.
Some marine sponges and most freshwater sponges also may reproduce asexually by forming gemmules. Gemmules are budlike structures that consist of a group of archaeocytes within a tough shell of spongin. Many gemmules are reinforced by spicules. Gemmules typically form in response to either cold or hot weather. Protected within the gemmule shell, the archaeocytes can survive periods of drought or freezing temperatures, though the parent sponge may die. Gemmules "hatch" when more favorable weather returns. The archaeocytes then spread out on a solid surface and develop into a new sponge.
Regeneration. The developmental abilities of archaeocytes give sponges remarkable powers of regeneration. Even if large parts of a sponge's body are lost or damaged, they may be replaced or repaired. In laboratory experiments, scientists have pressed sponges through extremely fine cloth so that the bodies of the sponges break up into separate cells or clumps of cells. When these cells are replaced in water, they first migrate together to form rounded cell clusters. Then the cell clusters reorganize to form complete sponges again.
Kinds of sponges
Sponges make up a phylum (major group) of animals called Porifera, which comes from a Latin word meaning pore-bearer. Zoologists divide sponges into three classes, based chiefly on common skeletal features.
Sponges with a limestone skeleton belong to the class Calcarea. Most species in this class inhabit shallow parts of oceans, but some have been found at depths of up to 13,000 feet (4,000 meters). The tiny sponge called Sycon belongs to this group.
A second class, called Hexactinellida, consists of marine sponges with a silica skeleton. These species are commonly called glass sponges. Their spicules form beautiful geometric patterns. Glass sponges live as deep as 23,000 feet (7,000 meters) beneath the ocean's surface. The Venus's-flower-basket is a typical kind of glass sponge.
All freshwater sponges and most of the best-known marine sponges are in the class Demospongiae. Most of these animals have a skeleton of silica or spongin or of both substances. One kind of sponge in this class, the boring sponge, bores into coral, seashells, and other hard structures. This activity helps shape such marine environments as coral reefs and seacoasts. Other marine species in this group of sponges include the red-beard sponge, the sheepswool sponge, and bath sponges.
Some ocean sponges have a skeleton of silica and spongin with a thick base of limestone. Scientists include these sponges, sometimes called coralline sponges, in either the class Calcarea or Demospongiae. Many coralline sponges live in underwater caves. They are closely related to marine sponges that lived hundreds of millions of years ago.
Scientific classification. Sponges make up the phylum Porifera, which is divided into the classes Calcarea, Demospongiae, and Hexactinellida.
Interesting facts about animals: an introduction to kingdom animalia
Kinds of animals.No one knows exactly how many kinds of animals there are. New kinds are found every year. So far, scientists have identified more than 1 1/2 million types of animals. About 1 million of these are insects. There are about 21,000 kinds of fish, 9,700 kinds of birds, 6,500 kinds of reptiles, 4,000 kinds of amphibians, and 4,500 kinds of mammals.
Largest ears and eyes.The largest ears of all animals are those of the African elephant. Elephant ears grow as large as 4 feet (1.2 meters) across. The largest eyes of all land animals are those of the horse and the ostrich. They measure about 1 1/2 times the size of human eyes.
The flying dragon is another name for the draco lizard. This lizard can spread out folds of skin to form "wings" that it uses to glide through the air from tree to tree. It lives in Asia and the East Indies.
Lives of animals range from several hours to many years. An adult mayfly survives only a few hours or days. Some giant tortoises have lived more than 100 years.
The world's only known poisonous bird is the hooded pitohui, which lives on the island of New Guinea. This brilliantly colored orange-and-black bird has poison on its feathers and skin. This poison serves as a defense against hawks, snakes, and other enemies. It is the same type of poison as that carried by the deadly poison-dart frog of South America.
The hummingbird can fly straight up like a helicopter. It can hover in front of a flower to suck the nectar. The bee hummingbird,which grows to only 2 inches (5 centimeters) long, is the smallest of all birds.
The chameleon's tongue is as long as its body. This lizard swiftly shoots out its tongue to capture insects for food. Certain chameleons can quickly change color and even develop spots and streaks that make them seem to be part of their background.
A tree-climbing crab lives on many tropical islands. It is called the coconut crab because it cracks coconuts with its powerful claws and eats the sweet meat.
The platypus,a mammal, has a bill like a duck and lays eggs as birds do. But it nurses its young with milk as do other mammals. It lives only on mainland Australia and the island of Tasmania.
Largest ears and eyes.The largest ears of all animals are those of the African elephant. Elephant ears grow as large as 4 feet (1.2 meters) across. The largest eyes of all land animals are those of the horse and the ostrich. They measure about 1 1/2 times the size of human eyes.
The flying dragon is another name for the draco lizard. This lizard can spread out folds of skin to form "wings" that it uses to glide through the air from tree to tree. It lives in Asia and the East Indies.
Lives of animals range from several hours to many years. An adult mayfly survives only a few hours or days. Some giant tortoises have lived more than 100 years.
The world's only known poisonous bird is the hooded pitohui, which lives on the island of New Guinea. This brilliantly colored orange-and-black bird has poison on its feathers and skin. This poison serves as a defense against hawks, snakes, and other enemies. It is the same type of poison as that carried by the deadly poison-dart frog of South America.
The hummingbird can fly straight up like a helicopter. It can hover in front of a flower to suck the nectar. The bee hummingbird,which grows to only 2 inches (5 centimeters) long, is the smallest of all birds.
The chameleon's tongue is as long as its body. This lizard swiftly shoots out its tongue to capture insects for food. Certain chameleons can quickly change color and even develop spots and streaks that make them seem to be part of their background.
A tree-climbing crab lives on many tropical islands. It is called the coconut crab because it cracks coconuts with its powerful claws and eats the sweet meat.
The platypus,a mammal, has a bill like a duck and lays eggs as birds do. But it nurses its young with milk as do other mammals. It lives only on mainland Australia and the island of Tasmania.
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