An animal is a multicellular eukaryotic organism belonging to the kingdom Animalia, also called Metazoa. Animals obtain organic nutrients from other organisms or their products rather than manufacturing them entirely from inorganic materials. Their cells lack cell walls, and most animals possess specialized tissues and can move during at least part of their life cycle. The kingdom includes sponges, jellyfish, worms, insects, molluscs, and vertebrates, including humans. In biology, “animal” therefore has a broader meaning than its everyday use for nonhuman mammals. (openstax.org)
Defining characteristics
Animal cells contain nuclei and other membrane-bound structures. Unlike the cells of plants, they are not enclosed by rigid cell walls. Cells adhere to one another and interact with extracellular materials, enabling coordinated development and function. In most groups, differentiated cells form tissues and organs; sponges instead have specialized cells without the conventional tissue organization found in most other animals. (openstax.org)
Animals are heterotrophs. Their feeding strategies include herbivory, predation, scavenging, filter feeding, and parasitism. Food supplies both building materials and chemical energy. Most animals ingest food and break it down through digestion, although some parasites absorb nutrients across their body surfaces. Movement, sensory responses, and coordinated activity commonly involve muscle cells and neurons, but these features are not universal: sponges lack both muscles and a nervous system. (openstax.org)
Classification and major groups
Animal taxonomy organizes species into nested categories, including phyla, classes, orders, families, and genera. Modern classification uses comparative anatomy, embryonic development, and molecular evidence, especially DNA sequences, to reconstruct evolutionary relationships. Similar appearance alone does not necessarily establish close ancestry, and classifications may change when new evidence becomes available. (openstax.org)
Major animal groups include:
- Porifera: sponges, whose porous bodies circulate water through feeding chambers.
- Cnidaria: jellyfish, corals, and sea anemones, characterized by specialized stinging cells.
- Mollusca: snails, clams, octopuses, and their relatives.
- Annelida: segmented worms, including earthworms and leeches.
- Arthropoda: insects, arachnids, crustaceans, and related animals with jointed appendages and an external skeleton.
- Echinodermata: sea stars, sea urchins, and related marine animals.
- Chordata: animals possessing a notochord and other characteristic structures during at least one developmental stage. (openstax.org)
Vertebrates are a subgroup of chordates and include fishes, amphibians, reptiles, birds, and mammals. “Invertebrate” is a collective term for animals without a vertebral column, not a single evolutionary lineage. Some chordates, such as tunicates and lancelets, are invertebrates. Arthropods account for an especially large share of described animal diversity. (openstax.org)
Body plans and development
Animal body plans differ in symmetry, tissue layers, internal cavities, segmentation, and arrangements of organs. Many sponges are asymmetrical. Cnidarians commonly have radial symmetry, with structures arranged around a central axis. Most other familiar animals belong to Bilateria, whose ancestral body plan has distinct left and right sides, a front and rear, and an upper and lower surface. Bilateral organization is often associated with directional movement and concentration of sensory structures at the front end. (openstax.org)
Embryonic tissue layers give rise to different adult structures. Bilaterian animals generally develop three principal layers: ectoderm, mesoderm, and endoderm. Their internal spaces may include a fluid-filled body cavity called a coelom. Developmental features helped establish the distinction between protostomes and deuterostomes; molecular evidence now supplements these comparisons. Adult echinoderms illustrate why classification cannot rely solely on outward symmetry: they commonly have radial organization but develop from bilaterally symmetrical larvae. (openstax.org)
Reproduction and life cycles
Sexual reproduction combines genetic material through the fusion of egg and sperm. Depending on the species, individuals have separate sexes or possess both male and female reproductive functions. Asexual reproduction also occurs, including budding, fragmentation, and parthenogenesis, in which an offspring develops from an unfertilized egg. Some species alternate between sexual and asexual reproduction under different conditions. (openstax.org)
Development may be direct, with young resembling adults, or indirect, with a distinct larval stage. Metamorphosis transforms larval structures into an adult body. In butterflies, for example, development proceeds through egg, caterpillar, pupa, and adult stages. Different diets and habitats at successive stages can reduce competition between juveniles and adults. (openstax.org)
Evolutionary history
Animal diversity reflects evolution from shared ancestors. Fossils and molecular comparisons provide complementary evidence: fossils document past organisms and environments, while molecular data help reconstruct relationships among surviving lineages. Their records have different limitations, so the timing and branching order of some early animal divergences remain subjects of research. (openstax.org)
Animals existed before the Cambrian explosion, the major diversification recorded more than half a billion years ago. During this interval, many major animal groups became conspicuous in the fossil record. The event was an extended evolutionary radiation, not an instantaneous appearance of modern species. Its interpretation involves interactions among developmental innovations, environmental conditions, and ecological relationships. (repository.si.edu)
Ecological roles and conservation
Animals occupy numerous positions in ecosystems. Herbivores consume primary producers; predators consume other animals; scavengers and detritus feeders use dead organic material. These relationships form interconnected food webs through which energy passes between organisms, with less energy available at successive feeding levels. (openstax.org)
Animal pollination and seed dispersal support plant reproduction. Insects, birds, and mammals transfer pollen between flowers, while fruit-eating animals may transport seeds away from parent plants. Such interactions connect animal populations with plant communities and contribute to biodiversity. (openstax.org)
Animal populations are affected by changes in land and sea use, direct exploitation, climate change, pollution, and invasive alien species. These pressures can alter abundance, distribution, and ecological interactions and increase the risk of extinction. Conservation research examines these drivers and their combined effects across terrestrial, freshwater, and marine environments. (files.ipbes.net)