Nonvertebrates

Nonvertebrates, or invertebrates, are animals that lack a vertebral column. They constitute the vast majority of animal life, with estimates that about 95–97% of described animal species are invertebrates. They occupy nearly every habitat, from ocean depths and freshwater to forests, deserts, and caves.

The major invertebrate groups include arthropods (insects, spiders, crustaceans, and myriapods), mollusks (snails, clams, squids, and octopuses), annelids (earthworms and leeches), cnidarians (jellyfish, corals, sea anemones), echinoderms (starfish, sea urchins, sea cucumbers), and sponges (porifera). Each group exhibits a wide range of forms and life histories, from microscopic larvae to large, complex organisms.

Anatomically, nonvertebrates display great diversity. Many have no internal skeleton; arthropods possess external exoskeletons, mollusks often have calcareous shells, and echinoderms have a calcareous internal skeleton. Reproductive strategies are likewise varied, including direct development, metamorphosis in insects and some mollusks, and a mix of fertilization modes.

Ecologically, nonvertebrates perform essential roles: pollination by insects, decomposition by worms and crustaceans, nutrient cycling, reef building by corals, and serving as prey or predators in food webs. They are also important in research and medicine, with many species providing insights and resources for science and industry.

Taxonomically, “nonvertebrate” is a descriptive term rather than a single clade; the group is defined by the absence of a backbone rather than by shared ancestry.