Invertebrates

Invertebrates are animals that lack a vertebral column or backbone. They include the vast majority of animal species and inhabit nearly every ecosystem. While they range in size from microscopic nematodes to massive squid, invertebrates share common features such as multicellularity, heterotrophy, and specialized tissues, though their body plans are highly diverse and often based on external skeletons or hydrostatic compartments.

The major invertebrate lineages include Arthropoda (insects, spiders, crustaceans, and millipedes), Mollusca (snails, clams, octopuses), Annelida (segmented worms), Cnidaria (jellyfish, corals, sea anemones), Echinodermata (starfish, sea urchins), Porifera (sponges), Platyhelminthes (flatworms), and Nematoda (roundworms). Insects within arthropods display extensive diversity and specialized life cycles. Many invertebrates have exoskeletons, while others rely on hydrostatic skeletons or internal supports.

Invertebrates occupy nearly all habitats, from deep oceans to deserts and from tropical forests to polar regions. They perform essential ecological roles: pollinating crops, decomposing organic matter, cycling nutrients, serving as food for other animals, and shaping habitats (as corals build reefs or burrowing worms modify soil). Reproduction is diverse, including sexual and asexual modes; metamorphosis in many insects and some marine groups is common.

In classification, "invertebrates" is a convenience rather than a single clade, encompassing many phyla with distinct evolutionary histories. The fossil record shows that animals first appeared in the oceans more than 550 million years ago, with invertebrate lineages dominating Earth for most of animal history. The study and conservation of invertebrates are important due to their ecological roles and sensitivity to environmental change.