Invertebrate — animals without a backbone

Characteristics and body plans

Invertebrates are defined by the absence of a vertebral column, but they show extreme diversity in anatomy and life cycles. Some groups have soft bodies (for example many molluscs), others are built from repeating segments (such as annelid worms), and many arthropods possess a rigid external structure. Insects and related groups do not have bones but typically have an external support called a skeleton on the outside of the body — an exoskeleton — that provides protection and attachment points for muscles. Other invertebrates rely on hydrostatic pressure, calcareous plates, or flexible tissues for support.

Major groups (common phyla)

• Arthropods — the largest invertebrate clade, including arachnids, crustaceans and insects.
• Molluscs — snails, slugs, clams and cephalopods such as octopus and squid, and bivalves.
• Echinoderms — sea stars (starfish), sea urchins and sea cucumbers.
• Cnidarians — jelly-like animals such as jellyfish and sea anemones.
• Porifera — sponges, simple filter-feeding animals with porous bodies.
• Platyhelminthes — flatworms, which include free-living and parasitic species.
• Nematodes (roundworms) and annelids (segmented worms) are important soft-bodied groups.
• Brachiopods and Bryozoa are sessile filter-feeders; some bryozoans superficially resemble corals.
• Tiny rotifers and other microscopic phyla expand the diversity of invertebrate life.

History and evolutionary context

Invertebrates represent the earliest-diverging animal lineages in the fossil record and today include both ancient and highly derived forms. Fossils from the Cambrian explosion demonstrate that many major body plans were established hundreds of millions of years ago. Because "invertebrate" is a term of convenience (any animal that is not a vertebrate), it groups together organisms that are not necessarily closely related; modern classification separates them into distinct phyla and clades based on anatomy, development and genetic data.

Ecological roles and human importance

Invertebrates perform essential functions: pollination by insects, decomposition by worms, reef-building by some cnidarians, nutrient cycling by molluscs and many others. Several invertebrate groups are economically important as food (certain molluscs and crustaceans), pests, disease vectors, or sources of scientific insight (model organisms such as fruit flies). Conservation of invertebrate diversity is increasingly recognized as critical for healthy ecosystems and human well-being.

Further reading and distinctions

Because the label "invertebrate" covers a wide array of life forms, more detailed information is organized by phylum and group. For general introductions and taxonomic lists see resources on arthropods (arthropods, insects), molluscs (molluscs, cephalopods), cnidarians (cnidarians) and poriferans (sponges). For a concise index of animal phyla and additional groups consult a curated list of animal phyla. See also specialized pages on skeleton types and exoskeletons for structural contrasts between invertebrates and vertebrates.

Related links: definition, jellyfish, sea anemone, starfish, sea urchin, sea cucumber, crustacean, arachnid, bivalve, brachiopod, bryozoan, coral.