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Unit 2 Lesson 5 Introduction to Animals

Copyright © Houghton Mifflin Harcourt Publishing Company


You Are an Animal!


• All animals are multicellular organisms, which means that they are made up of many cells.

• Animal cells are eukaryotic, so they have a nucleus. In animals, all of the cells work together to perform the life functions of the animal.

What characteristics do animals share?



• The cells of a multicellular organism develop into different kinds of cells in a process called differentiation.

• Some cells may become skin cells, and others may become gut cells.

• Each type of cell has a special function in an organism.




• Animals move in various ways. Some move to find food, shelter, and mates, while others move during only part of their life cycle.

• Most animals use sexual reproduction, in which a male sex cell, the sperm, fertilizes a female sex cell, the egg.

• The fertilized sex cell, or zygote, divides many times to form an embryo.




• Some animals, such as hydras and sponges, can reproduce asexually.

• Offspring of asexual reproduction are genetically identical to their parent.

• Animals cannot produce their own food, so they are consumers. A consumer is an animal that eats other organisms, such as plants, animals, or both, for energy.




• All animals need to maintain their bodies within a specific range of temperatures.

• Birds and mammals maintain their own body temperatures by using some of the energy released by chemical reactions.

• Other animals rely on their environment to maintain their body temperature.


Such Diversity!


What groups make up the diversity of animals?

• Animals are the most physically diverse kingdom of organisms.

• Animals can be categorized by symmetry or body plan.




• Some animals, such as sponges, are asymmetrical—you cannot draw a straight line to divide a sponge into equal parts.




• Animals like the sea anemone have a radial body plan, organized like the spokes of a wheel.

• Some animals have bilateral symmetry, with two mirror-image sides.


• Animals can also be categorized by internal traits, such as whether or not they have a backbone.

• An invertebrate is an animal without a backbone.

• Invertebrates do not have bones, but some have a hard, external covering, which supports the body, called an exoskeleton.

• Asexual reproduction is common in invertebrates.




• Two special kinds of invertebrates are tunicates and lancelets, which share some characteristics with vertebrates.




• Tunicates, such as sea squirts, are small, sac-shaped animals. Lancelets are small, fish-shaped animals.

• Tunicates and lancelets, along with vertebrates, are part of a group of animals called chordates.




• Animals with backbones are vertebrates. The backbone is part of an endoskeleton, an internal skeleton that supports an animal’s body.

• The backbone is made of bones called vertebraethat protect part of the nervous system. A vertebrate also has a braincase, or skull, that protects its brain.

• Almost all vertebrates reproduce sexually. In a few species, a female’s egg can develop into an individual without being fertilized.




Soft and Squishy?


What are some different kinds of invertebrates?

• Most animal species are invertebrates. A vast diversity of animals make up this group.




• Cnidarians live in oceans and have two body forms: polyp, like a sea anemone, or medusa, like a jellyfish.

• Porifera includes sponges with specialized cells connected by jelly-like material. They live on the ocean floor and filter food particles from water.




• Anthropoda includes animals that live on land and in water with jointed appendages and an exoskeleton that protects them from predators.

• Mollusca live in water or on land and have soft bodies. Many, such as snails and clams, have a protective outer shell and a muscular foot.

• Nematoda are roundworms that live in fresh water, soil, or other animals. Many of these animals, such as hookworms, are parasites.


Some Familiar Faces ...


What are some different kinds of vertebrates?

• Vertebrates are divided into five main groups: fish, amphibians, reptiles, birds, and mammals.

• Vertebrates live in water, on land, or both. They can eat plants, animals, or both.

• Both DNA and body form and structure are used to classify vertebrates.




• Amphibians live on land and in water. Most have four limbs and live near fresh water because their eggs and larvae need water to survive.

• Amphibians have thin skins that must be kept moist.

• Frogs, toads, and salamanders are examples of amphibians.




• Reptiles have bodies covered with scales or plates, and reproduce by laying eggs.

• Reptiles can live nearly anywhere on land because they can lay eggs out of water. The eggs are protected by membranes and a shell.

• Examples include turtles, snakes, lizards, and crocodiles.




• Birds have hollow bones, wings, and feathers. They lay eggs, which they sit on to keep warm. Most birds can fly. A few, such as penguins, cannot.

• Mammals have hair, a jaw, and three middle-ear bones, and they produce milk.

• Monotremes are mammals that lay eggs with shells.




• Marsupials have embryos that develop in a pouch. Placental mammals, such as wolves, beavers, and sloths, have embryos develop inside their bodies.

• Fish live in water. Cartilaginous fish, such as sharks and stingrays, have a skeleton made of flexible cartilage. Most fish are bony.

• Some fish reproduce by laying eggs. Other fish have embryos that develop inside the female.


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