Classifying Animals into Phyla Study Guide (page 2)
Animals are multicelled, usually highly mobile, and unable to produce their own food like plants can. The Animal Kingdom is divided into two large groupings, the invertebrates (animals with no internal skeleton, although they may have an external skeleton called an exoskeleton), and vertebrates (animals with an internal skeleton and a highly evolved nervous system, such as humans).
Because birds and mammals are so large, obvious, and similar to us, we tend to think of them as being the dominant animals on Earth. The real picture is one in which smaller, less obvious, and boneless creatures are dominant. Such creatures are plentiful and have adapted to environments we cannot easily visit, such as the ocean depths. These animals are in the large group we call the invertebrates. We, along with other mammals, birds, fish, reptiles, and amphibians, are vertebrates. All the vertebrates together make up less than 5% of all the animal species on Earth; invertebrates make up the rest.
It is conjectured that colonies of protists whose members became specialized to perform certain roles eventually developed into the earliest form of animals. Today, the sponges, which live mostly in saltwater, are the closest example of organisms that are a collection of single-celled creatures. They have no specialized organs or tissues but maintain a well-defined shape and have a very rudimentary skeleton.
What Is an Animal?
From sponges to human beings, wherever they are found, animals share some fundamental characteristics. When an animal egg is fertilized, it undergoes several cell divisions or cleavages, quickly producing a cluster of cells called a morula. As cleavage continues, the morula develops into several distinct stages, reaching a stage called the gastrula, which is a double-layered simple embryo. From this gastrula, the full, multicelled organism develops tissues and organ systems, and eventually develops into its adult form.
Immature as well as adult animals come in diverse forms. However, as multicellular animals, they all must come up with solutions to several basic problems, and these solutions give animals a different appearance from plants. The problems fall into several broad categories.
- Surface-area-to-volume issues: This issue is a matter of size. Nutrients, air, and water must be able to enter an animal's body to sustain life. This means that the surface area of an animal's body must be large enough to allow a sufficient amount of these elements to be used by the whole volume of the organism. In single-celled organisms, this means the cell size is strictly limited to the amount of nutrients that can pass through the cell membrane to support the whole volume of the cell. In multicelled organisms, specialized tissues and systems have evolved to bring in the necessary elements and then carry them to the cells. So it is not necessary for the body surface area of a large, multicelled animal to be able to supply all necessary elements. Specialized tissues and organ systems with very large surface areas have developed that absorb nutrients or air and filter wastes for the entire body. By working in conjunction with the circulatory system, these tissues and organs are able to support a large-sized (large-volume) multicelled body. The specialized tissues are found in the respiratory (breathing) system, urinary (excretory) system, and the digestive system.
- Body support and protection: All animals have some form of support and protection for their bodies. Sponges have a rudimentary skeletal network; crustaceans (such as crayfish) and insects have a hard outer coating called an exoskeleton; and mammals, birds, fish, reptiles, and amphibians have an internal skeleton. In all cases, these skeletal systems provide support to the animal's body and protect the internal organs from damage.
- Locomotion: Animals are heterotrophs and cannot produce their own food from exposure to sunlight, so they must acquire food. This need, as well as the need to mate and reproduce, forces an animal to move. Plants move but usually just in place, where they are rooted. Animals, on the other hand, move from place to place; this is called locomotion. Locomotion requires a muscular system, which animals came to develop in conjunction with the skeletal system to provide movement. Muscles are found as an adaptation only in animals, not in plants, fungi, or one-celled microorganisms.
- Sensory integration: Animals have complex bodies with many parts and systems that need coordination. This has resulted in the evolution of nervous tissue and, in many animals, a highly evolved nervous system, including a brain and spinal cord. In addition, animals have many specialized sensory organs (eyes, ears, noses, etc.) integrated into their nervous systems. These organs sense the environment and allow animals to show a very noticeable and marked response to environmental stimuli. The integration and coordination of sense organs, skeletal/muscular systems, and other bodily functions require an organized collection of specialized nervous tissue known as a central nervous system. The central nervous system has adapted into its most impressive form in human beings and other vertebrates.
Classifying Animals into Phyla
Phylum Porifera: sponges
- Collections of individual cells, with no tissues or organs, and no nervous system or skeleton.
Phylum Coelenterata: jellyfish, sea anemones, coral
- Usually very beautiful forms, their bodies are two-layered and symmetrical in a circular fashion with rudimentary organs and systems, but no skeleton.
Phylum Platyhelminthes: flatworms, tapeworms
- Their bodies are symmetrical in a left/right fashion (like humans). Their bodies have three layers and have very rudimentary nervous tissue.
Phylum Nematoda: roundworms
- They are symmetrical like the flatworms and have three body layers. Many are beneficial soil organisms, whereas some are parasites (such as hookworms and pinworms).
Phylum Annelida: segmented worms
- These have bodies similar to other previous worms but with some more advanced characteristics, including sensory organs and a relatively developed nervous system. Their bodies are divided into segments; earthworms are the best example of animals in this category.
Phylum Echinodermata: sea stars and sea urchins
- Their bodies have a circular symmetry with five body parts being arranged around a central axis. They have calcium spines or plates just under the skin and a unique water vascular system that is a series of fluid-filled vessels that provide body support and allow for locomotion.
Phylum Mollusca: snails, clams, and octopuses
- These have a well-developed circulatory system, nervous system, and digestive system; octopuses have particularly well-developed brains with highly maneuverable tentacles.
Phylum Arthropoda: crustaceans, spiders, and insects
- This phylum has more species than the other phylums, mostly because of all the insect species. Their bodies have exoskeletons, and most undergo metamorphosis (a transformation that allows them to grow by shedding their exoskeleton and developing into a larger or more adult form). They often have specialized body parts (antennae, pinchers, etc.), and they are well adapted to many environments.
Phylum Chordata: amphibians, reptiles, fish, birds, and mammals (including humans)
- These are the most familiar animals, and we all share four characteristics: a notochord that often develops into the vertebral column in vertebrates; a nerve cord that runs along our backs; gill slits at some point in our development; and a tail or at least a vestigial tail (humans have the tailbone or coccyx).
As multicellular animals, animals must come up with solutions to several basic problems. These solutions give animals a different appearance from plants. These problems fall into several broad categories: surface-area-to-volume issues, body support and protection, locomotion, and sensory integration. Animals are also organized into several phyla.
Practice problems of this concept can be found at: Classifying Animals into Phyla Practice Questions
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