Subject - General Science
Chapter : The Animal Kingdom

Invertebrates

Of the million or more animal species in the world, more than 98% are invertebrates. Invertebrates don’t have an internal skeleton made of bone. Many invertebrates have a fluid-filled, hydrostatic skeleton, like the jelly fish or worm. Others have a hard outer shell, like insects and crustaceans. There are many types of invertebrates. The most common invertebrates include the protozoa, annelids, echinoderms, mollusks and arthropods. Arthropods include insects, crustaceans and arachnids.

Protozoa

• Protozoa are simple, single-celled animals. They are the smallest of all animals. Most protozoa are microscopic in size, and can only be seen under a microscope. However, they do breathe, move and reproduce like multicelled animals.

• There are several types of protozoa. The amoebas are clear, shapeless cells. Flagellates have a body shape looking like a hair. Although we can’t see them, protozoa do a lot for us. Protozoa play a useful role in the food chain as a source of food for fish and other animals. Some protozoa are helpful to humans by eating dangerous bacteria. Unfortunately, other protozoa are parasites and can be harmful to humans by transmitting disease.

Worms and Leeches

• There are about 9,000 species of Annelids known today, including worms and leeches. They can be found almost anywhere in the world. Annelids have existed on Earth for over 120 million years.

• Annelids have bodies that are divided into segments. They have very well-developed internal organs. One common characteristic of annelids is that they don’t have any limbs.

• Some annelids may have long bristles. Others have shorter bristles and seem smooth, like the earthworm.

• There are many types of worms.Commonly known worms include earthworms, roundworms and flatworms. Most worms are small, measuring fractions of an inch to several inches long. Other worms, such as the ribbon worm, can grow up to 100 feet in length. Some worms are considered parasites, in that they live inside the human body.

Mollusks

• Mollusks were among the first inhabitants of the Earth. Fossils of mollusks have been found in rocks and date back over 500 million years. Mollusk fossils are usually well preserved because of their hard shell. Most mollusks have a soft, skin-like organ covered with a hard outside shell. Some mollusks live on land, such as the snail and slug. Other mollusks live in water, such as the oyster, mussel, clam, squid and octopus.

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Squid

• There are about 300 species of squid. They are native to most of the world’s oceans. The squid has a distinct head, eight arms and two tentacles. The mouth of the squid has a sharp horny beak used to kill and tear its prey into small pieces. The main body of the squid is enclosed in the mantle, which has a swimming fin along each side. However, the swimming fin is not the squid’s main way of moving through the water. The squid can suck water into the mantle and expel it out in a fast, strong jet. This jet propulsion provides fast, forward movement. Although most squid are less than 2 feet in length, the giant squid can grow up to 43 feet in length.

Cuttlefish

• Despite their name, the cuttlefish is not a fish, but a mollusk. The cuttlefish is native to all of the oceans of the world, but are more common in shallow coastal temperate and tropical waters. The cuttlefish has an internal shell or bone, called the cuttlebone, that helps them to be buoyant. Attached to this body structure is the head with eight arms and two feeding tentacles. The cuttlefish can easily camouflage itself by changing its skin color and pattern to blend in with its background. This helps the cuttlefish to hide from predators, and the sneak up on its prey. Like the squid and octopus, the cuttlefish can eject ink in an effort to escape from predators. This ink, called sepia, was once used as a dye to create ink used by artists.

Arachnids: Spiders, Ticks and Scorpions

• Arachnids are a type of arthropod. You know many of them as spiders. Common arachnids are spiders, scorpions, ticks and mites.

• Like other arthropods, the arachnids have a hard exoskeleton and jointed appendages for walking. Most arachnids have 4 pairs of legs. In some, the first pair of legs may be used for holding their prey and feeding. Unlike other arthropods, arachnids do not have antennae.

• Spiders are easily recognized with their 8 legs. All legs are used for walking. The first pair of legs is also used for holding prey and feeding. The second pair of legs may also be used for holding and killing their prey. Most spiders have 8 eyes. Spiders have fangs that are used to inject poison to paralyze-or kill their prey. Many spiders can produce silk threads to spin webs for catching prey, and for building an egg sack to hold and protect their eggs.

• Scorpions are large arachnids, some reaching over 8 inches in length. They have 4 pairs of legs, and a pair of pincers for catching and holding their prey. Scorpions also have a sharp stinger at the end of their tail that is used to paralyze or kill insects and small animals.Mites and ticks are small arachnids that are parasites living on the blood and tissue fluid of other animals. They can occasionally transmit disease. The abdomen also has appendages, such as legs, for crawling and swimming. Many crustaceans also have claws that help with crawling and eating.

Facts about Fish

• Fish are divided into three basic groups which include cartilaginous fish, bony fish, and lobe-finned fish. Fish were the first animals to evolve backbones.

• The ray-finned fish are the largest group of fish.

• Fish move by creating a wave motion that moves the length of its body.

• Fish are cold-blooded (ectothermic) animals.

• Many species of cichlids brood their eggs in their mouth.

• After the eggs hatch the parent continues to use their mouth to provide shelter for their young.

• Cartilaginous fish include the sea’s largest and most skilled marine predators.

• These include sharks, skates, rays, and chimeras. These fish have skeletons made from cartilage, not bone. The cartilaginous skeletons are more flexible than bone.

• The lateral line system on some fish detects variations in water pressure.

• This helps fish detect prey and avoid predators.

Facts About Reptiles

• There are about 8,000 species of known reptiles alive today.

• The first reptiles appeared approximately 340 million years ago during the Carboniferous Period.

• Reptiles are cold-blooded.

• Reptiles have scales.

• The Mesozoic Era is the ‘Age of Reptiles’.

• In many reptiles, the sex of the young is determined by the temperature the embryos are exposed to during incubation.

• Some of the largest reptiles alive today include the leatherback turtle, the Komodo dragon, and the saltwater crocodile.

Birds

• There are over 8,000 species of birds. Birds have 3 major differentiating characteristics: wings for flight, feathers, and a beak rather than teeth. Birds have adapted their vertebrate skeleton for flight. Their bones and skull are very thin, making their bodies extremely light. To support flight also required other changes to their skeleton. Obvious changes are the addition of wings. Other changes are less obvious. The claws and muscles of a bird’s foot are designed to lock and hold onto a perch even while the bird is sleeping.A bird’s respiratory system is also adapted to make it easier to breathe at high elevations, where air is thinner.

Mammals

• Mammals have several unique characteristics that differentiate them from other animals. Most mammals have hair, or fur, covering their body. They are also capable of regulating their body temperature. The mammals metabolism controls heat production, and the sweat glands help cool the body. These allow the mammal to maintain a constant body temperature, regardless of the environmental temperature. One other difference is that mammals give birth to fully formed babies, and the female mammals produce milk to feed their young. Most mammals walk on 4 legs, with only the humans walking upright on 2 legs. Aquatic mammals have flippers, or fins, for swimming rather than legs. Common mammals include: primates, such humans and monkeys; marsupials; rodents; whales; dolphins; and, seals

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