Classifying Animals Diversity of Animals Animals can be classified and grouped based on similarities in their characteristics. Animals make up one of the major biological groups of classification. All animals are multicellular organisms. Since they cannot make their own food, animals must consume other organisms in order to receive energy. Most animals also have the ability to move and undergo sexual reproduction. The major classification group including all animals can be further divided into vertebrates and invertebrates. Invertebrates Invertebrates are animals that do not have a backbone, or any other types of bones. However, many invertebrates do have an exoskeleton, or a hard outer skeleton, for support. A snail at the beach, an insect in a tree, and an earthworm in the soil are all kinds of invertebrates. The invertebrate group is made up of a variety of organisms, including sponges, starfish, octopuses, snails, insects, earthworms, crustaceans, jellyfish, and coral. The invertebrate group can also be divided into many smaller classification groups. For example, two major groups of invertebrates are mollusks and arthropods. Octopuses and snails are both kinds of invertebrates that are classified as mollusks. Most mollusks have a hard shell they can pull into to protect the soft parts of their bodies. They usually have a muscular foot that allows movement or can open and close their shell. Snails, clams, slugs, squid, and octopuses are all examples of mollusks. and fleas), and spiders. Ants and lobsters are both kinds of invertebrates that are classified as arthropods. Arthropods are invertebrates that have an exoskeleton, a segmented body, and jointed legs. This group includes crustaceans (like lobsters and crabs), insects (like ants
Vertebrates Vertebrates are animals that have backbones. They include a variety of organisms that can be further divided into smaller classification groups. These groups are fish, amphibians, reptiles, birds, and mammals. Clownfish and sharks are classified as fish. Fish are cold-blooded vertebrates that live in water. They obtain dissolved oxygen from the water through their gills. Most fish also lay eggs and have fins and scales. Salmon, clownfish, tuna, sharks, trout, snapper, and swordfish are all classified as fish. Frogs and salamanders are classified as amphibians. Amphibians are cold-blooded vertebrates. Most amphibians can breathe underwater with gills when they are young and then undergo metamorphosis at a certain age. Most adult forms breathe air using lungs, but many can also "breathe" through their skin. Frogs, toads, and salamanders are the three major kinds of amphibians. Snakes, turtles, and crocodiles are classified as reptiles. Reptiles are cold-blooded, have scales or plates on their bodies, and have lungs that breathe air. Most reptiles also lay eggs and live at least part of their lives on land. Examples of reptiles include snakes, lizards, turtles, and crocodiles. Ducks, ostriches, and parrots are all vertebrates that are classified as birds. Birds are vertebrates that breathe air with lungs, are warmblooded, lay hard-shelled eggs, and have feathers, two feet, two wings, and toothless beaks. Examples of birds include parrots, eagles, penguins, hawks, chickens, ducks, turkeys, and ostriches. Dolphins, elephants, and wolves are all vertebrates that are classified as mammals. Mammals are warm-blooded vertebrates that breathe air with lungs, have sweat glands, produce milk to feed their young, and have fur or hair. Most mammals also have babies that are born live. Humans, bears, dogs, cats, elephants, dolphins, whales, squirrels, foxes, giraffes, rabbits, and wolves are all examples of mammals.
Characteristics of Animals An animal is a living thing that eats other organisms for food and can move from one place to another for at least part of its life. Animals are classified, or divided into groups, based on similarities and differences. Animals can differ from one another in the way that they are shaped or in the way that they reproduce, respire, digest food, or circulate blood. Reproduction One of the differences among different kinds of animals is the way they reproduce. Some produced through asexual reproduction, whereas others reproduce through sexual reproduction. There are advantages and disadvantages to both sexual and asexual reproduction. The lifestyle of an organisms is related to how and when it reproduces. Asexual Reproduction Asexual reproduction is the easiest form of reproduction. Organisms do not have to use energy to find mates because only one parent is required. The offspring are generally identical to parents, and there is usually very little variation of characteristics within the population. So, it is more difficult for organisms that reproduce asexually to adapt to the changes in their ecosystems. Asexual reproduction is usually linked to the simplest animals, like sponges and hydras. However, there are a few vertebrates that reproduce asexually. In this case, the parents and offspring are all females. Examples of asexual reproduction in animals include: budding - sponges and hydras fragmentation - flat worms parthenogenesis - whiptail lizards Sexual Reproduction Most animals reproduce sexually. Sexual reproduction usually requires two parents. However, some animals are hermaphroditic, which means they produce both sperm and eggs. Animals that produce both sex cells have a higher chance of finding a mate than animals with separate sexes because any animal of the same species that they encounter is a potential mate. Other animals, like humans, have separate female and male organisms within the species. Sex cells produced by the females must combine with sex cells produced by the males in a process called fertilization. During fertilization, egg cells and sperm cells fuse to produce an offspring. Fertilization can be internal or external.
Internal fertilization egg and sperm combine inside of an organism usually takes place in the female mammals, reptiles, birds, some amphibians and fish External fertilization takes place outside of an organism most amphibians and fish Gas Exchange (Respiration) All animals need oxygen to survive. There are a few methods animals can use to obtain oxygen. The main methods of gaining oxygen are as follows: 1. Integumentary exchange - gases diffuse across the skin or body covering. This is used by flatworms, earthworms, and amphibians. 2. Tracheal respiration - animals with hard coatings have small holes in their body covering. An empty tube runs through the body covering and into the tissue that needs the oxygen. Oxygen passes through the tube and is absorbed into the tissue. Carbon dioxide is released from the tissue and passes back out of the tube. This is used by spiders and some insects. 3. Internal gills - internal gills are at the back of the mouth and allow gas exchange between water and blood vessels. Internal gills are used by fish. 4. External gills - external gills project out of the animal and into the water. External gills are used by some insects and some amphibians. 5. Lungs - a lung is an internal respiratory surface in the shape of a sac or cavity. Oxygen from the air is absorbed into blood vessels, and carbon dioxide from blood vessels is released into the air. Lungs are used by mammals, birds, and reptiles. Some fish and amphibians also use lungs as well as other methods.
Body Plan Animals have different body plans that relate to how they live. Similarity in body plans is usually a sign that organisms are related. Some examples of body plans include: Asymmetry Asymmetry means that an organism does not have symmetry. Asymmetric bodies do not have regular, predictable shapes, and it is not possible to cut through a central axis to create similar left and right halves. This is the simplest type of body plan. The sea sponge above has an irregular body and its left and right sides are not similar. This is a classic example of asymmetry. Radial Symmetry Radial symmetry is a type of symmetry in which several planes divide an organism into almost identical parts. Organisms with radial symmetry have no "left" or "right" sides, but they do have a "top" and a "bottom". Radially symmetric objects have more than one line of symmetry. The starfish above has five lines of symmetry. Since more than one line of symmetry exists, the starfish is radially symmetrical. Bilateral Symmetry Bilateral symmetry is a kind of symmetry in which a single plane divides an object into halves that are roughly mirror images of each other. The red line indicates a line of symmetry. The parts on either side of the line are roughly identical. Bilateral symmetry is an adaptation for locomotion. Organisms with asymmetry or radial symmetry either move very slowly or do not move at all for most of their lives. One of the most important features to develop as part of bilateral symmetry is the head. The head is the result of nervous tissue and sensory organs that are concentrated on the end of the organism that faces the direction of locomotion. This development gave organisms a chance to sense the environment towards which they were moving. This is advantageous to find food and avoid predators.
Digestion All living things require energy to grow, develop, and reproduce. Animals accomplish this by consuming other organisms. However, the organisms consumed must to be broken down to access the raw materials. In simple animals, such as sponges, digestion takes place within individual cells. Sponges do not possess true tissues or organs. So, digestion takes place at the cellular level. Other organisms have what is called an incomplete digestive system. It is also referred to as a twoway digestive tract. Organisms with incomplete digestive systems have a single opening through which food enters; wastes are expelled from the same opening. Jellyfish and flatworms display this type of digestive system. Most animals possess a complete digestive system, or a one-way digestive tract. These animals have a separate opening for food, the mouth, and wastes are expelled through the anus. So, food travels in one direction and does not mix with wastes. Vertebrates, insects, and mollusks have complete digestive systems. Circulation As organisms become larger and more complex, they require a system of transferring gases and nutrients to all parts of the body. The circulatory system is responsible for this function in animals. The simplest is an open circulatory system. Insects have open circulatory systems. Blood and body fluid mix to form a substance called haemolymph. Since this system is not very efficient, the size of these organisms is limited. Closed circulatory systems are the most efficient at transporting nutrients and gases to all parts of the body. Earthworms are the simplest animals with a closed circulatory system. They possess one dorsal and one ventral blood vessel. Aortic arches are portions of the vessels that act like hearts by pumping the blood. Unlike insects and other organisms with an open circulatory system, the blood does not mix with other body fluids. All vertebrates have closed circulatory systems with true hearts. Fish have the simplest heart. It contains two chambers: an atrium, through which blood enters the heart, and a ventricle, through which the blood exits to the aorta. The vertebrate heart continued to evolve to become more efficient. New chambers that helped separate nutrient-rich and nutrient-poor blood evolved.