invertebrate

An invertebrate is an animal that lacks a vertebral column. Invertebrates represent the largest and most diverse group of animals, comprising the vast majority of animal species and inhabiting a wide range of environments, from deep oceans to deserts and freshwater systems.

The term covers many phyla and body plans. Major groups include Arthropoda (insects, spiders, crustaceans, millipedes and centipedes), Mollusca (gastropods, bivalves, cephalopods), Annelida (segmented worms), Cnidaria (jellyfish, corals, sea anemones), and Echinodermata (starfish, sea urchins, sea cucumbers), along with smaller phyla such as Nematoda and Platyhelminthes. Unlike vertebrates, most invertebrates lack an internal skeleton composed of bone or a spine, though many have exoskeletons or hydrostatic skeletons.

Invertebrates exhibit a wide range of life histories and adaptations. They may reproduce sexually or asexually, and some undergo metamorphosis. Their feeding strategies are diverse, including herbivory, predation, scavenging, filter feeding, and parasitism. Sensory and nervous systems similarly vary from simple nerve nets to complex brains in certain mollusks and other groups.

Ecologically, invertebrates perform crucial roles as pollinators, decomposers, prey, and keystone species in many ecosystems. They also hold economic and scientific importance as pests, food sources, and model organisms; notable examples include the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, as well as highly studied cephalopods for their behavioral complexity.