invertebraten

Invertebraten, commonly called invertebrates, are animals that lack a vertebral column. They represent the majority of animal species and occupy nearly every habitat, from the deepest oceans to soils, freshwater to deserts. The term encompasses a broad range of body plans and life histories, from microscopic organisms to large, complex animals.

The major invertebrate phyla include Arthropoda (insects, spiders, crustaceans), Mollusca (snails, clams, squids), Annelida (segmented worms), Cnidaria (jellyfish, corals, sea anemones), Echinodermata (starfish, sea urchins), and Porifera (sponges). Arthropods are the largest and most diverse group, typically with segmented bodies, jointed limbs, and, in many species, an exoskeleton. Mollusks show wide variation, including shelled snails and highly modified forms such as octopuses. Echinoderms exhibit pentaradial symmetry as adults. Sponges have simple organization and porous bodies.

Anatomy and development among invertebrates vary greatly. Some possess complex nervous systems and organs, while others rely on simple nerve nets or diffuse nerve cells. Skeletons range from rigid external shells to internal hydrostatic systems. Reproduction is mostly sexual, with eggs and sperm often released into the water, but several groups reproduce asexually or have life cycles with larval stages that differ markedly from adults.

Invertebrates play essential ecological roles as pollinators, grazers, predators, scavengers, and prey. They contribute to nutrient cycling, soil formation, and energy flow in ecosystems. Humans interact with invertebrates as pests, sources of food, and subjects of scientific study, and their study informs biology, ecology, and evolutionary theory. The fossil record shows a long, diverse history dating back hundreds of millions of years.