Invertebrata, a negative terra in zoology, employed by Lamarck to designate animals destitute of a vertebral column or backbone. Exclusive of the protozoa, these constitute three out of the four great divisions of the animal kingdom, viz., articulates, mollusks, and radiates ; the remaining division consists of the vertebrates, or those having an internal skeleton with a backbone for its central support, including man and other mammals, birds, reptiles, amphibians, and fishes. The articulates, characterized by a jointed body, include insects, arachnids, centipedes, crustaceans (as crabs and lobsters), and worms; the mollusks are those generally denominated shell-bearing animals; the radiates include the echinoderma (or sea urchins, star fishes, and holothurians), the aca-lephs or jelly fishes, and the polyps (like hydra, actinia, and the coral animals). There is no homology or affinity between the structural type of the vertebrates and invertebrates, though there may be analogy; for instance, the head of an insect is not homologous with the head of a man, a bird, a reptile, or a fish, as it has no distinct brain cavity nor cranial vertebrae, yet its sense organs and other parts perform the same functions.
Aristotle distinguished invertebrates from vertebrates, calling the former (bloodless) and the latter (having blood); Oken made the same distinction in his gut animals and flesh animals, and Ehren-berg in his ganglioneura and myeloneura; even Lamarck was aware that in his inverte-brata all the organs are contained in a single cavity, while in the vertebrata there are distinct cavities for the nervous system and the organs of vegetative life. Lamarck divided the invertebrata into two orders and twelve classes, viz.: apathetic animals, with the five classes of infusoria, polypi, radiaria, tunicata, and vermes; and sensitive animals, with the seven classes of insects, arachnids, Crustacea, annelids, cirripeds, conchifera, and mollusks; all distinguished from vertebrata, or intelligent animals. The development of the embryo and the methods of reproduction in the invertebrates are different from those of the vertebrates. In the radiates the germ surrounds the yolk like a crust, from which the more animated parts are derived, the alimentary canal being formed from the central mass; reproduction may also take place by buds or by transverse division in the polyps and jelly fishes, the latter also presenting the curious phenomena of alternate generation.
In articulates the embryo is formed at the lower part of the yolk, with its dorsal surface toward the latter, so that the yolk is enveloped from below upward, the uniting suture being upon the back. In mollusks the yolk is introduced from the lower side of the animal, as in vertebrates, but there is no upper cavity for the nervous system, as in the latter. It is thus evident that the term invertebrata is not equivalent in zoological precision to, and is far more comprehensive than, the vertebrate division; the oyster, the butterfly, the star fish, all invertebrates, have nothing in common but the absence of a vertebral column. Invertebrates include by far the most numerous and diversified forms in the animal kingdom; in them we find many important physiological questions answered, and by them we understand otherwise inexplicable problems of animal life and of its relations to changes in the earth's surface; in them we see a circulation of blood without a heart or without distinct vessels, respiration effected by a vascular integument, the nervous system reduced to its essential elements of ganglia with connecting cords, the external skeleton enclosing the muscles and organs, the plant-like mode of reproduction and of true hermaphroditism, and the multiplication of organs independently performing the same functions (as digestive sacs, gills, locomotive appendages, &c). The different classes will be described more fully in their respective order.
The whole subject is most learnedly treated by Prof. Owen in his "Lectures on the Invertebrate Animals " (1843).