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Skip to main content. While we can easily identify dogs, birds, fish, spiders, and worms as animals, other organisms, such as corals and sponges, are not as easy to classify. Animals vary in complexity, from sea sponges to crickets to chimpanzees, and scientists are faced with the difficult task of classifying them within a unified system.
They must identify traits that are common to all animals as well as traits that can be used to distinguish among related groups of animals. The animal classification system characterizes animals based on their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup. This classification scheme is constantly developing as new information about species arises. Understanding and classifying the great variety of living species help us better understand how to conserve the diversity of life on earth.
Even though members of the animal kingdom are incredibly diverse, most animals share certain features that distinguish them from organisms in other kingdoms. All animals are eukaryotic, multicellular organisms, and almost all animals have a complex tissue structure with differentiated and specialized tissues. Most animals are motile, at least during certain life stages. All animals require a source of food and are, therefore, heterotrophic: ingesting other living or dead organisms.
This feature distinguishes them from autotrophic organisms, such as most plants, which synthesize their own nutrients through photosynthesis. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites. Most animals reproduce sexually with the offspring passing through a series of developmental stages that establish a fixed body plan. The body plan refers to the morphology of an animal, determined by developmental cues. Heterotrophs : All animals are heterotrophs that derive energy from food.
The a black bear is an omnivore, eating both plants and animals. The b heartworm Dirofilaria immitis is a parasite that derives energy from its hosts. It spends its larval stage in mosquitoes and its adult stage infesting the heart of dogs and other mammals. Animals, besides Parazoa sponges , are characterized by specialized tissues such as muscle, nerve, connective, and epithelial tissues. In addition, animals possess unique tissues, absent in fungi and plants, which allow coordination nerve tissue and motility muscle tissue.
Animals are also characterized by specialized connective tissues that provide structural support for cells and organs. This connective tissue constitutes the extracellular surroundings of cells and is made up of organic and inorganic materials.
In vertebrates, bone tissue is a type of connective tissue that supports the entire body structure. The complex bodies and activities of vertebrates demand such supportive tissues. Epithelial tissues cover, line, protect, and secrete; these tissues include the epidermis of the integument: the lining of the digestive tract and trachea.
They also make up the ducts of the liver and glands of advanced animals. The animal kingdom is divided into Parazoa sponges and Eumetazoa all other animals. Although they do possess specialized cells that perform different functions, those cells are not organized into tissues. These organisms are considered animals since they lack the ability to make their own food.
When we think of animals, we usually think of Eumetazoans, since most animals fall into this category. Sponges : Sponges, such as those in the Caribbean Sea, are classified as Parazoans because they are very simple animals that do not contain true specialized tissues. The different types of tissues in true animals are responsible for carrying out specific functions for the organism.
This differentiation and specialization of tissues is part of what allows for such incredible animal diversity. This allows animals to survive in environments where they must compete with other species to meet their nutritional demands. Most animals undergo sexual reproduction and have similar forms of development dictated by Hox genes. Most animals are diploid organisms their body, or somatic, cells are diploid with haploid reproductive gamete cells produced through meiosis. The majority of animals undergo sexual reproduction.
This fact distinguishes animals from fungi, protists, and bacteria where asexual reproduction is common or exclusive. In other protists, glassy silica-based shells or pellicles of interlocking protein strips encase the cells. The pellicle functions like a flexible coat of armor, preventing the protist from external damage without compromising its range of motion.
Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Protists that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts.
Other protists are heterotrophic and consume organic materials such as other organisms to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole.
The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome, which breaks down the food particle into small molecules that diffuse into the cytoplasm for use in cellular metabolism. Undigested remains ultimately exit the cell via exocytosis. Protist metabolism : The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.
Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists function as mixotrophs, obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available. The majority of protists are motile, but different types of protists have evolved varied modes of movement.
Protists such as euglena have one or more flagella, which they rotate or whip to generate movement. Paramecia are covered in rows of tiny cilia that they beat to swim through liquids. Other protists, such at amoebae, form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a surface, and pull themselves forward. Some protists can move toward or away from a stimulus; a movement referred to as taxis.
Protists accomplish phototaxis, movement toward light, by coupling their locomotion strategy with a light-sensing organ. Different types of motility in protists : Protists use various methods for transportation. Protists live in a wide variety of habitats, including most bodies of water, as parasites in both plants and animals, and on dead organisms. Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect different host species at different developmental stages to complete their life cycle.
Some protists are unicellular in the haploid form and multicellular in the diploid form, which is a strategy also employed by animals. Other protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations that is also used by plants. The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types.
Plasmodial slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of slime during their feeding stage. The slime mold glides along, lifting and engulfing food particles, especially bacteria.
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