Invertebrata, a collective title for those animals which agree in not exhibiting the characteristics of Vertebrates—viz. a dorsal nerve cord, a dorsal median supporting axis or notochord, respiratory clefts on the pharynx, a ventral heart, and eyes arising for the most part as outgrowths of the brain. But the dividing line is no longer so clear as it once seemed, for not only are Ascidians or Tunicata recognised as degenerate Vertebrate or Chordate animals, but several 'worm' types, among Nemerteans and Chaetopods, approach Vertebrates in some of their characters, while Balanoglossus (q.v.) and Cephalodiscus (q.v.) are so near the boundary line that they are usually called Hemi-chordata or half Vertebrates.
Invertebrate animals are first divided into (1) Protozoa—uni-cellular—and (2) Metazoa—multi-cellular. The latter then fall into two distinct divisions; (a) without body-cavities—Sponges and Cœlenterates—and (b) with more or less of a body-cavity—the Cœlomata. Among the latter star-fish, &c. (without including Vertebrates), the Echinoderms, the Arthropods (Crustaceans, Insects, &c.), and the Molluscs (bivalves, snails, cuttle-fish), and finally a great mob of 'worms,' divisible into many classes—Flat-worms, Nemerteans, Round-worms, Chaetopods, &c.—have to be distinguished. See VERTEBRATA, and separate articles.