Ninety-seven percent of animals have no backbone
"Invertebrate" is a convenience label, not a true taxon: animals that never form a vertebral column. One estimate puts them at about ninety-seven percent of animal species, spanning ten-micrometre myxozoans to colossal squid nine to ten metres long. Most sit in Protostomia, yet the group is paraphyletic once tunicates and lancelets are counted.
The word joins Latin in- ("without") to vertebra, a turning joint of the spine. Unlike Arthropoda or Vertebrata, Invertebrata lacks cladistic force—tunicates and cephalochordates are closer to vertebrates than to snails or insects. Still, zoologists keep the term for everything outside Vertebrata. Familiar body plans include sponges, cnidarians, flatworms, nematodes, annelids, echinoderms, molluscs, and arthropods. Support may be hydrostatic, as in jellyfish and worms, or a hard outer shell, as in insects and crabs.
IUCN tallies cite about 66,178 described vertebrate species, implying over ninety-five percent of named animals are invertebrates, with insects by far the largest slice. Shared traits beyond "no backbone" are thin: heterotrophy, usually differentiated tissues (sponges excepted), and often a gut with one or two openings. Symmetry runs radial, bilateral, or spherical—or fails, as in many gastropods' helical shells and the oversized claw of fiddler crabs.
Nervous systems fire to trauma, heat, and pH shifts; the medicinal leech yielded the first identified invertebrate neuron, and sea-hare work links nociceptors to learning. Terrestrial arthropods pipe gases through spiracles and tracheae rather than oxygenated blood. Reproduction is wildly varied: roughly ten percent of oribatid mite species are thought to have reproduced asexually for more than four hundred million years. Sponges draw water through pores; cnidarians and comb jellies keep a single gut opening and only two main germ layers.
Social behaviour appears in cockroaches, termites, ants, bees, spiders, and more; eusocial colonies are the extreme. Model species Drosophila and Caenorhabditis elegans unlocked meiosis and reproduction biology that extends to incredible invertebrate diversity beyond the lab. Molluscs and annelids—second-largest phyla by species—were long linked by trochophore larvae but now seem convergent rather than close kin.
Source: Invertebrate
