Table of Contents
Position of Animals Within the Eukaryotes
Within the domain Eukarya, animals (kingdom Animalia, often called Metazoa) form a natural group of multicellular, heterotrophic organisms that feed by ingestion. They are distinguished from the other eukaryotic kingdoms (Protista, Plantae, Fungi) by a characteristic combination of traits:
- Multicellularity with a relatively fixed body plan (at least in adults)
- Lack of rigid cell walls
- Heterotrophic nutrition by ingestion and internal digestion
- Development from a characteristic early embryonic stage (the blastula)
- Predominantly active movement at some life stage
- Complex cell–cell adhesion and communication systems unique to animals
Animals are most closely related to a group of unicellular or colonial protists called choanoflagellates; this relationship is important for understanding how multicellular animals evolved.
General Characteristics of Animals
Although there are exceptions and specializations, most animals share the following features:
- Eukaryotic, multicellular organization
Cells are organized into tissues, and in most groups into organs and organ systems. Many animal cells are highly specialized (muscle cells, nerve cells, blood cells, etc.). - No cell wall
Animal cells lack cellulose or chitin cell walls. Instead they are supported by: - A flexible cell membrane
- Extracellular matrix (ECM) rich in collagen and other structural proteins
- In many groups, a skeleton (hydrostatic, exoskeleton, or endoskeleton)
- Heterotrophy by ingestion
Animals obtain organic molecules by consuming other organisms or organic material and digesting it in an internal cavity (gut) or within cells. This contrasts with: - Plants: mostly photoautotrophic
- Fungi: heterotrophic by absorption, often after external digestion
- Active movement
Most animals can move actively at least during some life stage, often using specialized contractile muscle cells and coordinated by a nervous system. - Nervous and muscle tissues
True muscle and nerve tissues are a defining innovation of animals (with some simple groups having only rudimentary forms). These allow rapid responses, coordinated movement, and behavior. - Reproduction and life cycles
- Predominantly sexual reproduction with haploid gametes and a diploid-dominant life cycle
- Characteristic early development:
- Zygote → cleavage → blastula (hollow ball of cells)
- In “higher” animals: gastrulation to form primary germ layers
- Many groups also show a variety of asexual reproductive strategies and complex life cycles with metamorphosis.
Major Structural Themes in Animals
Symmetry and Body Axes
Animal body plans can be classified by their symmetry:
- Asymmetry
No regular body symmetry (e.g., many sponges). - Radial symmetry
Body parts arranged around a central axis; multiple planes can divide the animal into similar halves (e.g., many cnidarians like jellyfish). Often associated with a sedentary or free-floating lifestyle. - Bilateral symmetry
A single plane divides the body into left and right mirror-image halves. Defines: - Anterior–posterior (head–tail) axis
- Dorsal–ventral (back–belly) axis
- Typically associated with directed movement and cephalization (concentration of sense organs and nervous tissue at the anterior end).
Most animal phyla and almost all familiar animals are bilaterians.
Germ Layers and Tissue Organization
Embryonic development establishes the basic tissue architecture:
- Diploblastic animals
- Two primary germ layers: ectoderm and endoderm
- Typically radial symmetry; simple tissues (e.g., cnidarians such as sea anemones and jellyfish).
- Triploblastic animals
- Three germ layers: ectoderm, mesoderm, endoderm
- Mesoderm forms muscles, many organs, and, in many groups, body cavities and support structures.
- Almost all bilaterian animals are triploblastic.
Triploblastic animals differ in how (and whether) a fluid-filled body cavity (coelom) is formed within the mesoderm; this has important consequences for movement and organ arrangement but is not identical with “complexity.”
Segmentation (Metamerism)
In some animal groups, the body is organized into repetitive units (segments):
- Seen clearly in annelids (segmented worms), arthropods (insects, crustaceans, etc.), and, in a modified form, in vertebrates (repeated vertebrae, ribs, and muscle segments).
- Segmentation allows:
- Specialization of body regions (tagmatization, e.g., head–thorax–abdomen in insects)
- More flexible and efficient movement
Segmentation has evolved more than once; not all “segmented” organs indicate common ancestry.
High-Level Classification of Animals
Systematics of animals uses both morphological and molecular data. At a broad scale, animals are often divided into a few large clades. Names and ranks (phylum, subkingdom, etc.) can vary, but some major groups are widely recognized.
Basal Animal Lineages
- Porifera (sponges)
- Mostly asymmetrical, lacking true tissues and organs
- Filter feeders with specialized cell types but relatively simple organization
- Important for understanding the earliest steps of animal multicellularity
- Radiata (paraphyletic concept; primarily Cnidaria and sometimes Ctenophora)
- Cnidaria: jellyfish, sea anemones, corals
- Diploblastic, radial symmetry (though many have some directional features)
- Specialized stinging cells (cnidocytes)
- Ctenophora: comb jellies
- Eight rows of ciliary plates used for locomotion
- Often bioluminescent
- These groups show the emergence of tissues, simple nerve nets, and body axes.
(The exact branching order of sponges, ctenophores, and cnidarians is still debated.)
The Bilateria
The majority of animal diversity belongs to the Bilateria—bilaterally symmetrical, triploblastic animals. Two especially large clades are:
Protostomes
In protostomes (in a classical sense), the embryonic blastopore often becomes the mouth, and certain patterns of early development are characteristic. Major protostome groups include:
- Lophotrochozoa (term based on shared larval forms or feeding structures)
- Platyhelminthes (flatworms)
- Often dorsoventrally flattened
- Many free-living and parasitic forms
- Annelida (segmented worms)
- True segmentation, closed circulatory system in many
- Mollusca
- Snails, bivalves, cephalopods, and others
- Typically with a muscular foot, mantle, and (often) shell
- Ecdysozoa (animals that grow by molting an exoskeleton)
- Arthropoda
- Insects, arachnids, crustaceans, myriapods
- Segmented body, jointed appendages, chitinous exoskeleton
- Extremely successful and diverse group, dominating terrestrial and many aquatic habitats
- Nematoda
- Roundworms, many microscopic
- Pseudocoelomate body, tough cuticle, immense ecological and medical importance
Protostomes exhibit enormous diversity in body form, habitats, and life strategies, from tiny interstitial worms to complex social insects.
Deuterostomes
In deuterostomes, the blastopore typically becomes the anus, and the mouth forms secondarily; they also share characteristic developmental and molecular traits. Major deuterostome groups include:
- Echinodermata
- Sea stars, sea urchins, sea cucumbers, etc.
- Adults often have (penta)radial symmetry, but larvae are bilaterally symmetrical
- Unique water vascular system with tube feet for movement, feeding, and gas exchange
- Chordata
- Defined by four key features (at least at some life stage): notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail
- Includes:
- Invertebrate chordates (e.g., tunicates, lancelets)
- Vertebrates (fish, amphibians, reptiles, birds, mammals), many of which are the best-known animals to humans
Diversity of Habitats and Life Strategies
Animals occupy nearly every environment on Earth, from deep-sea trenches to high mountains, and from deserts to polar ice. Some general patterns:
- Aquatic vs. terrestrial life
- Early animals were marine; numerous groups later colonized freshwater and land.
- Terrestrial life required adaptations for:
- Water balance and desiccation resistance
- Support and locomotion in air
- Gas exchange outside water
- Reproduction and development away from water (e.g., amniotic egg in reptiles, birds, mammals)
- Free-living vs. parasitic lifestyles
- Many lineages contain both free-living and parasitic representatives.
- Parasitism has evolved repeatedly and is associated with:
- Simplification or modification of body structures
- Complex life cycles and host switching
- Strong coevolution with hosts
- Sessile vs. mobile forms
- Some animals are permanently attached as adults (e.g., many sponges, corals), while their larvae are motile.
- Many animals alternate between stationary and motile phases, with different ecological roles.
- Feeding strategies
- Filter feeders (e.g., sponges, many bivalves)
- Grazers and browsers (e.g., many herbivorous insects and mammals)
- Predators (numerous groups)
- Scavengers and detritivores
- Specialized symbionts relying on mutualistic microbes (e.g., ruminant mammals, some insects)
Animals in Ecological Systems
Although details of energy flow and nutrient cycles are treated elsewhere, some aspects are characteristic for animals as a group:
- Consumer roles
- Animals function as primary consumers (herbivores), secondary and higher-level consumers (carnivores), and detritivores.
- They mediate energy transfer between trophic levels and strongly influence plant, microbial, and other animal populations.
- Ecosystem engineers
- Many animals modify their habitats (e.g., coral reef builders, earthworms, beavers, termites), creating conditions for many other species.
- Symbiotic interactions
- Mutualism (e.g., pollinators and flowering plants, gut symbionts)
- Commensalism (e.g., remoras riding on sharks)
- Parasitism and parasitoidism (e.g., various worms, arthropods, protozoan symbionts of animals)
These interactions contribute to the shaping of ecosystems and to coevolutionary processes across many lineages.
Systematics and Phylogeny of Animals
Modern animal systematics reconstructs evolutionary relationships (phylogeny) using:
- Comparative morphology and embryology (body plans, developmental patterns)
- Molecular data (DNA, RNA, protein sequences)
- Fossil evidence to calibrate major divergences and trace body-plan innovations
Key insights include:
- Monophyly of animals
All multicellular animals share a common ancestor, forming a single natural clade. - Early divergence of simple lineages
Sponge-like and other simple groups separated early from the lineage leading to more complex animals. - Single origin of Bilateria
Bilaterally symmetrical, triploblastic animals show deep genetic and developmental homologies, suggesting an ancient bilaterian ancestor. - Repeated evolution of similar traits
Complex features such as eyes, flight, and segmentation have both homologous and convergent origins in different animal lineages.
Naming and ranking higher animal taxa (e.g., whether to use categories like “subkingdom” or “superphylum”) are matters of convention, but the underlying evolutionary relationships, especially among major clades such as Bilateria, Lophotrochozoa, Ecdysozoa, and Deuterostomia, are central for understanding animal diversity.
Significance of the Animal Kingdom
Animals are central to:
- Ecosystem functioning: as consumers, pollinators, decomposers (indirectly), and ecosystem engineers.
- Human societies: as sources of food, materials, companionship, disease vectors, and model organisms in biological research.
- Evolutionary studies: providing some of the best-documented examples of adaptation, speciation, and complex behavior.
Within the broader context of systematics, the kingdom Animalia illustrates how a single evolutionary origin of multicellularity, combined with subsequent diversification of body plans and lifestyles, has generated one of the most morphologically and ecologically diverse groups within the domain Eukarya.