Animals are composed of many cells, the basic units of structure and function in living things. The cells of most animals are organized into higher levels of structure, including tissues, organs, and systems. A tissue is a group of similar cells that perform a speciﬁc function. Tissues may combine to form an organ. An organ is a group of different tissues that performs a more complex function than each tissue could perform alone. In most animals, organs combine to form systems.
The major functions of animals are to obtain food and oxygen, keep internal conditions stable, move, and reproduce. Structures or behaviors that allow animals to perform these basic functions in their environments are called adaptations.
Most animals produce offspring by sexual reproduction,the process by which a new organism develops from the joining of two sex cells. The joining of an egg cell and a sperm cell is called fertilization. Some animals reproduce by asexual reproduction, in which an organism produces an identical copy of itself.
Biologists have deﬁned 35 major groups of animals, each of which is called a phylum (plural: phyla). The evolutionary process has resulted in all the different phyla that exist today. Animals are classiﬁed according to how they are related to other animals. These relationships are determined by the animal's body structure, the way the animal develops, and its DNA. One important structural characteristic used to classify animals is whether or not the animal has a backbone. Animals without backbones make up most of the animal species on Earth. These animals are called invertebrates. Animals with backbones are called vertebrates.
The balanced arrangement of parts, called symmetry, is characteristic of many animals. An animal with bilateral symmetry has a line of symmetry that divides it into halves that are mirror images. Animals with radial symmetry have many lines of symmetry that all go through a central point. A few animals, including most sponges, have no symmetry.
Depending on their symmetry, animals share some general characteristics. Animals with radial symmetry have body parts that are arranged in a circular pattern around a central point, like spokes on a bicycle wheel. They do not have distinct front or back ends. Animals with radial symmetry live in water and do not move very fast. They either stay in one spot or creep along the bottom.
Animals with bilateral symmetry are often larger than those with radial symmetry. They have more complex and streamlined bodies. These streamlined bodies allow the animals to move more quickly than animals with radial symmetry. The sense organs of animals with bilateral symmetry are located at the front end of their bodies. Swift movement and sense organs help animals with bilateral symmetry obtain food and avoid enemies.
Sponges and cnidarians are animals that live in water. Sponges are invertebrate animals that usually have no body symmetry and never have tissues or organs. Sponges stay in one place, attached to hard surfaces. The body of a sponge is something like a bag that is covered all over with openings called pores. Water enters these pores and carries food and oxygen into the sponge. Water also carries the sponge's waste products away.
Sponges reproduce both asexually and sexually. Budding is one form of asexual reproduction. In budding, small new sponges grow from the sides of an adult sponge. These young sponges eventually break off to live on their own. In sexual reproduction, water currents carry sperm from one sponge into the pores of another sponge. These sperm fertilize the sponge's egg cells. After fertilization, a larva develops. A larva is an immature form of an animal that looks very different from the adult.
Jellyﬁshes, sea anemones, and corals are cnidarians, invertebrates that have stinging cells and take food into a central body cavity. Cnidarians use stinging cells to capture food and defend themselves.
There are two cnidarian body plans, polyp and medusa.A polyp, such as a sea anemone, is shaped something like a vase, with the mouth opening at the top. In contrast, a medusa, such as a jellyﬁsh, is shaped like a bowl, with a mouth that opens downward. Some cnidarians are polyps at one point in their lives and medusas at another point. Other cnidarians are either a polyp or medusa for all their lives.
A cnidarian captures its food by using its stinging cells. The stinging cells release a spined structure that snares the prey. Some stinging cells also release venom. Cnidarians reproduce both asexually and sexually. For polyps, budding is the most common form of asexual reproduction.
Some cnidarians live in a colony, a group of many individual animals. Stony corals and the Portuguese man-of-war are examples of colonies of cnidarians. At the beginning of its life, a free-swimming coral larva attaches to a solid surface. The coral polyp reproduces asexually, and its offspring reproduce too. The polyp produces a hard skeleton around its soft body. Over time, the animals die but their hard skeletons remain. Eventually, these mounds of skeletons form a coral reef.Coral reefs are home to more species of ﬁshes and invertebrates than any other environment on Earth.
There are many kinds of worms. Biologists classify worms into three major phyla-ﬂatworms, roundworms, and segmented worms. Worms have long, narrow bodies without legs, and they have bilateral symmetry. They have tissues, organs, and body systems. Worms are the simplest animals to have a brain. Worms reproduce both sexually and asexually.
Flatworms include planarians, ﬂukes, and tapeworms. Flatworms are ﬂat and as soft as jelly. Many ﬂatworms are parasties.
A parasite is an organism that lives inside or on another organism. The parasite takes its food from its host, the organism in or on which it lives. There are also free-living organisms, such as planarians, that do not live in or on other organisms. Planarians are scavengers,which means they feed on dead or decaying material. A tapeworm is a kind of parasitic ﬂatworm. Its body is adapted to absorbing food from its host's digestive system.
Roundworms have cylindrical bodies. Unlike cnidarians or ﬂatworms, roundworms have a digestive system that is like a tube, open at both ends. Food enters through the mouth. Wastes leave the digestive system through the anus, an opening at the other end of the tube.
Earthworms and other segmented worms have bodies made up of many linked sections called segments. Some organs are repeated in most segments. But other organs, such as the reproductive organs, occur only once.
Segmented worms have a closed circulatory system.In a closed system, such as your own, the blood ﬂows only through connected tubes called blood vessels. A closed system moves the blood around an animal's body more quickly than an open circulatory system can.
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