Invertebrates

INVERTEBRATES (Evertebrata), the aggregate of all animals that lack a vertebral column; invertebrates are contrasted with vertebrates (Vertebrata), which possess an internal skeleton in the form of a series of vertebrae on the dorsal side of the body. Corresponding approximately to Aristotle's division of animals into bloodless (invertebrates) and blooded (vertebrates), this grouping of the animal world no longer has strict scientific significance. Lamarck first introduced the division of animals into invertebrates and vertebrates, but later, beginning with Cuvier, animals came to be divided into a significantly larger number of major groups called types. All these types, except the last one, are included among the invertebrates. The last type, the chordates (Chordata), contains all true vertebrates as well as several small groups of marine animals in which the notochord exists either only in the embryonic period or persists in the adult state, but only as a vestige. Before the works of Alexander Kovalevsky, these lower chordate animals were classed among the invertebrates, but now they are united with them, as stated, into a single type. There are disagreements regarding the classification of invertebrates. The most generally accepted division is into the following seven types. Protozoa form a separate type, which includes all unicellular animals. Many forms of pathogenic significance belong to them. All other animals are united under the name of multicellular animals (Metazoa). Until recently, the existence of a special type, Mesozoa, occupying an intermediate position between unicellular and multicellular animals, was recognized, but at present part of the Mesozoa are assigned to protozoa; the rest are regarded as degraded Metazoa. Recently, the division of multicellular animals into two major branches has become very widespread: radial (Radiata) and bilateral (Bilateria). Radial animals have a lower organization and are divided into two types—sponges (Spongia, or Porifera) and coelenterates (Coelenterata). Some of the sponges (badjaga) and coelenterates (horny corals containing iodine) have been used in medicine. The remaining four types of invertebrates and the type of chordates belong to the group of bilaterians. The first of these types encompasses worms (Vermes) and contains many important parasites of humans and domestic animals (the classes of flukes, tapeworms, roundworms, acanthocephalans, and leeches). The second type, mollusks (Mollusca), contains almost no forms of medical importance. However, some freshwater mollusks serve as intermediate hosts for fluke worms (for example, liver flukes) that parasitize humans and other animals. The third type, arthropods (Arthropoda), is connected in its origin with one group of worms, the annelids. A multitude of ectoparasites (and more rarely endoparasites) belong to this type. Such are lice, fleas, bedbugs, ticks, and others. A number of venomous animals also belong here, such as the tarantula, karakurt, scorpions, centipedes, and others. Finally, many arthropods are important as transmitters of various parasitic infections. Various mosquitoes and sandflies transmit the causative agents of malaria, yellow fever, and also certain roundworms (filariae). Lice are vectors of typhus. Small copepods serve as intermediate hosts for a number of human worms (guinea worm, broad fish tapeworm), and so on. The last type of bilaterians, echinodermates (Echinodermata), in the adult state often mask their bilateral structure with a five-rayed arrangement of organs.