Animal
Animals are multicellular, eukaryotic organisms that belong to the biological kingdom Animalia, distinguished by their heterotrophic nutrition—ingesting organic matter from external sources rather than producing it themselves—lack of cell walls, presence of specialized tissues such as nervous, muscle, connective, and epithelial types, and typically motile lifestyles at some life stage.[1] They reproduce primarily through sexual means involving gamete fusion, though some species also employ asexual methods like budding or parthenogenesis, and their embryonic development often includes a blastula stage leading to diverse body plans that may be asymmetrical, radially symmetrical, or bilaterally symmetrical.[1] The kingdom Animalia encompasses an immense diversity of life forms, with nearly 2 million species described as of 2025 and estimates of 5 to 8 million additional species remaining to be described, the vast majority of which are invertebrates.[2] Arthropods, including insects, spiders, and crustaceans, represent the most species-rich phylum, accounting for about 84% of all known animal species with over 1.2 million described.[3] Animals are classified into approximately 35 phyla based on body plan complexity, including the number of germ layers (two in simpler diploblasts like sponges and cnidarians, three in more complex triploblasts), the presence and type of body cavity (acoelomate, pseudocoelomate, or eucoelomate), and developmental patterns such as protostomy (mouth develops first, as in mollusks and arthropods) or deuterostomy (anus develops first, as in echinoderms and chordates).[4] Major phyla include Porifera (sponges), Cnidaria (jellyfish and corals), Platyhelminthes and Nematoda (flatworms and roundworms), Annelida (segmented worms), Mollusca (snails, octopuses, and clams), Arthropoda, Echinodermata (starfish and sea urchins), and Chordata (which includes vertebrates like mammals, birds, reptiles, amphibians, and fish).[5] Animal evolution originated in the oceans more than 600 million years ago from unicellular protist ancestors, with the group's diversification accelerating during the Cambrian Explosion around 530 million years ago, when most major phyla appeared in the fossil record.[2] This radiation marked the emergence of complex multicellularity, predation, and ecological interactions that shaped modern ecosystems, leading to animals' colonization of terrestrial, freshwater, and aerial environments over subsequent geological periods.[6] Today, animals play critical roles in global biodiversity, serving as pollinators, decomposers, prey, and predators, while facing threats from habitat loss and climate change that underscore their interconnectedness with other kingdoms of life.[2]Etymology and Definition
Etymology
The word "animal" derives from the Latin noun animal, a nominal use of the adjective animalis meaning "having breath" or "soul," which stems from anima ("breath, soul, life").[7] This etymology reflects an ancient conception of animals as living beings distinguished by respiration and vitality, entering English in the early 14th century to denote any sentient creature, including humans. In Roman texts, animal was employed to translate and adapt Greek biological concepts, emphasizing entities capable of motion and sensation as opposed to inert matter or plants.[8] In ancient Greek philosophy and science, the equivalent term was zōon (ζῷον), signifying a "living being" or "animal," broadly encompassing creatures with life and movement.[9] Aristotle, in works such as Historia Animalium (History of Animals), used zōon to classify and describe a wide array of organisms, grouping them into hierarchies based on shared traits like reproduction and locomotion, while distinguishing them from plants (phuton) through their capacity for self-initiated motion.[9] This Greek framework influenced Roman scholars like Pliny the Elder, who in Naturalis Historia adopted animal to categorize mobile, breathing life forms within the natural world, laying groundwork for later taxonomic systems.[10] The term evolved significantly in modern biology following Carl Linnaeus's Systema Naturae (1735), where "Animalia" was established as one of three kingdoms—alongside Vegetabilia (plants) and Mineralia—encompassing multicellular organisms characterized by motility, ingestion of organic matter, and nervous systems, explicitly including humans as Homo sapiens. This Linnaean system formalized the distinction of animals from plants (lacking locomotion) and minerals (inanimate), promoting a hierarchical binomial nomenclature that standardized "animal" as a biological category rather than a purely philosophical one. A key etymological shift occurred in the 19th century amid advances in microscopy and evolutionary theory, when unicellular protozoans—initially termed "animalcules" and classified under Animalia—were excluded from the kingdom.[11] Pioneered by naturalists like Carl Theodor von Siebold in 1845, who redefined Protozoa as a subkingdom of unicellular animals, the category was later separated by Ernst Haeckel in 1866 with the introduction of Protista for organisms blurring animal-plant boundaries, reflecting Darwinian insights into gradual evolutionary transitions.[11] By the late 19th century, "Animalia" narrowed to Metazoa (multicellular animals), excluding protozoans to emphasize complex, differentiated structures.[12]Definition and Scope
Animals are defined as multicellular, eukaryotic organisms that belong to the biological kingdom Animalia, characterized by their heterotrophic mode of nutrition, whereby they obtain energy by consuming organic material from other organisms rather than producing it through photosynthesis.[13] Unlike plants and algae, animals lack chlorophyll and cell walls, relying instead on ingestion or absorption of pre-formed organic compounds, which distinguishes them from autotrophic kingdoms.[14] This core definition encompasses a vast array of life forms, from simple sponges to complex vertebrates, all sharing these fundamental traits that enable active acquisition of resources.[15] Key criteria for inclusion in the animal kingdom include the absence of rigid cell walls, which allows for flexibility and motility in most species, and the typical presence of a nervous system for coordinating responses to environmental stimuli, though exceptions like sponges (phylum Porifera) lack true nervous tissues yet are still classified as animals due to their multicellularity and other shared features.[13] Animals are excluded from other kingdoms such as Plantae (which possess chlorophyll and cell walls of cellulose) and Fungi (which have chitinous cell walls and often absorb nutrients externally), as well as from the diverse group of mostly unicellular eukaryotes known as protists, which are now classified into several distinct supergroups within the domain Eukarya rather than a single kingdom.[14][16] This delineation ensures that only organisms fitting the multicellular, heterotrophic, and eukaryotic profile without photosynthetic or cell-walled structures are included, providing clear boundaries for taxonomic classification.[15] Modern refinements to the definition of animals have emerged from molecular biology, placing them within the holozoan clade—a monophyletic group of eukaryotes that includes animals (Metazoa) and their closest unicellular relatives, such as choanoflagellates, filastereans, and ichthyosporeans.[17] Choanoflagellates, in particular, are considered the sister group to animals based on genomic and phylogenetic analyses, sharing key genetic features like cadherin and tyrosine kinase genes that predate multicellularity.[18] These molecular insights refine the scope by highlighting evolutionary continuity with unicellular holozoans, while maintaining the exclusion of more distant lineages like fungi and plants, thus anchoring the animal kingdom in a robust phylogenetic framework.[19]Characteristics
Structural Features
Animals are multicellular eukaryotes characterized by the presence of specialized cells organized into tissues and organs, enabling complex functions such as movement, digestion, and sensory perception. This multicellularity arises from a common ancestor and allows for division of labor among cell types, distinguishing animals from unicellular protists. In most animals, excluding sponges (Porifera) and placozoans, cells differentiate into two or three primary germ layers during embryonic development. Diploblastic animals, such as cnidarians (e.g., jellyfish and corals), possess only ectoderm and endoderm; the ectoderm forms the outer covering and nervous tissue, while the endoderm lines the digestive cavity.[20][21] Triploblastic animals, which include the majority of animal phyla like bilaterians (e.g., arthropods, mollusks, and chordates), develop an additional mesoderm layer between the ectoderm and endoderm. The mesoderm gives rise to muscles, connective tissues, and circulatory components, enhancing structural complexity and support for active lifestyles.[20][22] Support structures in animals provide rigidity, protection, and a framework for muscle attachment, varying by habitat and body plan. Exoskeletons are external, rigid coverings secreted by the epidermis, commonly found in arthropods such as insects and crustaceans, where they are composed primarily of chitin—a tough polysaccharide that offers defense against predators and environmental stresses while allowing flexibility at joints.[23][24] In contrast, endoskeletons are internal frameworks, as seen in echinoderms (e.g., sea urchins with calcareous ossicles) and vertebrates (e.g., humans and fish with bone and cartilage). Vertebrate endoskeletons, made of mineralized tissues like hydroxyapatite-embedded collagen in bone, grow with the organism and facilitate efficient muscle leverage for locomotion.[23][25] Some animals, like earthworms, rely on hydrostatic skeletons using fluid-filled coelomic cavities for support and movement.[26] Sensory and nervous systems in animals range from diffuse networks to highly centralized structures, enabling detection of environmental stimuli and coordinated responses. Basal animals like cnidarians exhibit simple nerve nets—decentralized meshes of interconnected neurons that facilitate basic reflexes such as contraction in response to touch, without a central brain.[27][28] In more derived bilaterians, nervous systems centralize into ganglia or brains; for instance, flatworms have anterior nerve clusters, while vertebrates possess a dorsal central nervous system with a brain encased in a cranium and a ventral nerve cord, processing complex sensory inputs from eyes, ears, and mechanoreceptors.[29][27] Circulatory systems transport nutrients, gases, and wastes, with adaptations reflecting metabolic demands and body size. Open circulatory systems, prevalent in arthropods and most mollusks, involve a heart pumping hemolymph into body cavities (hemocoel), where it bathes tissues directly before returning to the heart, suiting lower-pressure needs in smaller or less active animals.[30] Closed circulatory systems, found in annelids, cephalopods, and vertebrates, confine blood within vessels, maintaining higher pressure for efficient delivery to distant tissues, as in the multi-chambered hearts of mammals.[31][32] Respiratory adaptations complement these by facilitating gas exchange; aquatic animals like fish use gills—vascularized filaments that countercurrent exchange oxygen from water, maximizing uptake efficiency. Terrestrial vertebrates employ lungs, invaginated sacs with alveoli for air breathing, supported by diaphragms or rib movements to drive ventilation.[33][34]Development and Life Cycle
Animal development begins with fertilization, where a sperm cell fuses with an egg cell to form a zygote, initiating embryonic growth in sexually reproducing species.[35] This process restores the diploid chromosome number and activates the zygote's metabolic machinery.[36] Following fertilization, cleavage occurs as the zygote undergoes rapid mitotic divisions without significant cell growth, producing a multicellular blastula composed of smaller blastomeres.[37] The blastula stage features a fluid-filled cavity called the blastocoel, which facilitates subsequent rearrangements.[38] Gastrulation follows cleavage, marking a critical reorganization where cells migrate and differentiate to form the three primary germ layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer).[36] These germ layers establish the basic body plan and give rise to all major tissues and organs in the adult animal.[37] Neurulation then ensues, primarily in chordates but with analogous processes in other animals, involving the folding of the ectoderm to form the neural tube, which develops into the central nervous system.[39] This stage ensures the proper positioning of neural structures along the dorsal side of the embryo.[40] Animal life cycles exhibit two main patterns: direct development, where the embryo develops into a miniature adult without a distinct larval phase, as seen in mammals where viviparous or oviparous offspring resemble adults from hatching or birth; and indirect development, featuring a free-living larval stage that differs morphologically from the adult, common in insects and amphibians.[41] In indirect cycles, the larva often serves ecological roles like dispersal or feeding before transforming into the reproductive adult form.[42] These variations reflect adaptations to environmental pressures, with direct development minimizing exposure to predation during vulnerable early stages.[43] Metamorphosis represents a profound transformation in indirect developers, driven by hormonal signals that trigger tissue remodeling and growth. In arthropods, the steroid hormone ecdysone, derived from cholesterol, initiates molting and metamorphic changes by binding to nuclear receptors that regulate gene expression for histolysis and histogenesis.[44] Juvenile hormone modulates the timing, preventing premature adult features during larval instars, while declining levels allow ecdysone to promote adult differentiation.[45] This process ensures the larva's specialized structures are replaced by those suited to the adult's lifestyle, such as wings in insects.[46] Aging and senescence in animals involve progressive decline in physiological function, culminating in increased mortality risk, with telomere shortening playing a key role in limiting cell replication in multicellular organisms.[47] Telomeres, repetitive DNA sequences at chromosome ends, erode with each division due to incomplete replication, eventually triggering replicative senescence and genomic instability.[48] Programmed cell death, or apoptosis, contributes to senescence by eliminating damaged cells, maintaining tissue homeostasis but accelerating overall aging when dysregulated.[49] In multicellular contexts, these mechanisms balance repair and turnover, though they ultimately lead to organismal deterioration.[50]Reproduction
Animals exhibit a wide array of reproductive strategies, primarily asexual and sexual, which enable adaptation to diverse ecological niches and maximize fitness under varying conditions. Asexual reproduction produces genetically identical offspring, allowing rapid colonization without the need for mates, while sexual reproduction promotes genetic diversity through gamete fusion, enhancing adaptability to changing environments. These modes often coexist in the same species, with the choice influenced by environmental stability and population density.[51] Asexual reproduction in animals includes mechanisms such as budding, fragmentation, and parthenogenesis. In sponges (phylum Porifera), asexual reproduction commonly occurs through budding, where small outgrowths develop into functional individuals that detach from the parent, or via fragmentation, in which body pieces regenerate into complete sponges. Freshwater sponges additionally form gemmules—dormant clusters of cells encased in protective coatings—that germinate under favorable conditions to produce new individuals. Budding is also prevalent in cnidarians like hydra (Hydra spp.), where a polyp-like bud emerges from the parent's body wall, grows, and separates as a clone, facilitating quick population expansion in stable aquatic habitats. Parthenogenesis, the development of unfertilized eggs into viable offspring, is observed in certain reptiles, such as whiptail lizards (genus Aspidoscelis), which consist entirely of females that produce diploid eggs through premeiotic endoduplication, resulting in clonal daughters.[52][53][54][55] Sexual reproduction in animals involves gametogenesis—the meiotic production of haploid sperm and eggs—followed by fertilization, which restores diploidy. Many animals are hermaphroditic, possessing both male and female reproductive organs, as seen in earthworms (family Lumbricidae), which are simultaneous hermaphrodites that align during mating to exchange sperm packets, enabling cross-fertilization despite self-compatibility. In contrast, dioecious (gonochoristic) species maintain separate sexes, such as most vertebrates, where males produce sperm and females ova. Fertilization can be external, as in many aquatic animals like amphibians and fish, where gametes are released into the surrounding water to meet, increasing dispersal but risking predation; or internal, prevalent in terrestrial groups like reptiles, birds, and mammals, where males deposit sperm directly into the female's reproductive tract via copulation, enhancing zygote protection.[56][57][58] Mating behaviors in animals often include elaborate courtship rituals to signal fitness, synchronize reproduction, and reduce mating errors. These displays vary widely, such as the vibrational signals and dances in insects like fruit flies (Drosophila spp.) or the plumage fanning and calls in birds like peacocks (Pavo cristatus), which attract females by demonstrating health and genetic quality. Parental care further diversifies reproductive investment; while many species provide minimal post-fertilization attention, mammals exemplify high parental involvement through viviparity, where embryos develop internally nourished by the mother via a placenta, followed by lactation and prolonged guarding, as in humans and elephants, which boosts offspring survival rates.[59][60] Evolutionary trade-offs in reproduction are captured by r/K selection theory, which posits a continuum of strategies balancing reproductive output against offspring quality. r-selected species, thriving in unpredictable environments, prioritize high fecundity and early maturity with low parental investment, exemplified by insects like mosquitoes that produce thousands of eggs but offer little care. Conversely, K-selected species, adapted to stable, resource-limited habitats near carrying capacity (K), invest in fewer offspring with extensive care, as in large mammals like whales, which bear single young after long gestations and provide years of protection. This framework, formalized by Pianka in 1970, highlights how density-dependent selection shapes life-history traits to optimize fitness.[61]Diversity
Size and Morphology
Animals display an extraordinary range in body size, spanning over seven orders of magnitude, from the minute fairyfly Dicopomorpha echmepterygis at 0.139 mm in body length to the colossal blue whale (Balaenoptera musculus), which attains a maximum confirmed length of approximately 30 m.[62][63] This disparity underscores the evolutionary flexibility of animal form, where size influences physiological constraints and ecological niches. Allometric scaling laws govern how traits such as metabolic rate and biomass vary with body mass; for instance, Kleiber's law posits that basal metabolic rate scales as mass raised to the power of 3/4 across diverse taxa, reflecting optimizations in resource allocation and energy use.[64] These scaling relationships also predict biomass distribution within populations, where larger species maintain lower densities to balance energetic demands.[65] Morphological diversity in animals manifests through distinct body plans adapted to functional needs. Bilateral symmetry predominates in most phyla, enabling cephalization and efficient locomotion, as seen in vertebrates and arthropods. In contrast, echinoderms like sea stars exhibit pentaradial symmetry in adulthood, facilitating omnidirectional interaction with the environment despite bilateral larval stages.[21] Annelids, such as earthworms, feature metameric segmentation, dividing the body into repeated units that enhance flexibility, burrowing capability, and regenerative potential. These adaptations arise from underlying structural features like germ layers but diverge widely to suit specific lifestyles. Allometric principles further illuminate how size shapes morphology through surface-to-volume ratios. Small animals benefit from high ratios, promoting rapid diffusion of oxygen and nutrients across body surfaces without specialized transport systems, as in microscopic planktonic larvae. Larger animals, however, face diminishing ratios, compelling the evolution of circulatory systems and respiratory organs to counteract diffusion limitations, which can constrain maximum body size in oxygen-poor environments.[66] Extreme size variations highlight environmental influences on morphology. Deep-sea gigantism affects invertebrates like the colossal squid (Mesonychoteuthis hamiltoni), reaching over 10 m, potentially due to cold temperatures slowing metabolism and allowing prolonged growth amid sparse resources.[67] Conversely, insular dwarfism reduces body size in large mammals on resource-limited islands; for example, prehistoric Cypriot dwarf hippopotamuses (Phanourios minor) weighed under 200 kg, a fraction of mainland relatives, as an adaptation to caloric scarcity.[68]Major Phyla and Distribution
The animal kingdom encompasses over 30 phyla, with eight major phyla accounting for the vast majority of described species diversity: Porifera, Cnidaria, Platyhelminthes, Nematoda, Arthropoda, Mollusca, Echinodermata, and Chordata. These phyla exhibit distinct body plans, symmetries, and adaptations that reflect their evolutionary divergence. Porifera, or sponges, are simple, sessile, filter-feeding organisms lacking true tissues or organs, with asymmetrical or radial symmetry and a porous body structure for water flow. Cnidaria, including jellyfish, corals, and sea anemones, feature radial symmetry, a gastrovascular cavity for digestion, and cnidocytes—specialized stinging cells for prey capture and defense. Platyhelminthes, the flatworms, display bilateral symmetry, a flattened acoelomate body, and often hermaphroditic reproduction, with many species parasitic. Nematoda, or roundworms, have a pseudocoelomate, unsegmented cylindrical body covered by a flexible cuticle, enabling their presence in diverse microhabitats. Arthropoda, encompassing insects, crustaceans, and arachnids, are characterized by a chitinous exoskeleton, jointed appendages, and segmentation, supporting their extraordinary mobility and adaptability. Mollusca, such as snails, squids, and bivalves, possess a soft body with a muscular foot for locomotion, a mantle for shell secretion, and often a radula for feeding. Echinodermata, including starfish and sea urchins, exhibit pentaradial symmetry in adults, a calcareous endoskeleton with spines, and a unique water vascular system for locomotion and feeding. Chordata, which includes vertebrates and some invertebrates like tunicates, are defined by a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some life stage, often with a coelom and advanced sensory systems.[69][70]| Phylum | Key Traits | Estimated Described Species |
|---|---|---|
| Porifera | Asymmetrical/radial symmetry; no true tissues; porous canal system; sessile filter feeders | ~6,000[69] |
| Cnidaria | Radial symmetry; cnidocytes; gastrovascular cavity; polyp/medusa forms | ~10,000[69] |
| Platyhelminthes | Bilateral symmetry; acoelomate; flat body; often parasitic | ~15,000–20,000[69] |
| Nematoda | Bilateral symmetry; pseudocoelomate; cylindrical body with cuticle; unsegmented | ~28,000 (millions estimated undescribed)[71][72] |
| Arthropoda | Bilateral symmetry; exoskeleton; jointed appendages; segmented body | >1,000,000 (largest phylum, ~80–90% of all animal species)[69] |
| Mollusca | Bilateral symmetry; coelomate; muscular foot; mantle; often shelled | ~100,000[69] |
| Echinodermata | Radial symmetry (adults); water vascular system; spiny endoskeleton | ~7,000[69] |
| Chordata | Bilateral symmetry; notochord; dorsal nerve cord; pharyngeal slits | ~65,000[73] |