Platyhelminthes

Platyhelminthes

A phylum of the invertebrates, commonly called the flatworms. They are bilaterally symmetrical, nonsegmented, dorsoventrally flattened worms characterized by lack of coelom, anus, circulatory and respiratory systems, and exo- or endoskeleton. They possess a protonephridial excretory system, a complicated hermaphroditic reproductive system, and a solid mesenchyme which fills the interior of the body (see illustration). Three classes occur in the phylum: (1) the Turbellaria, mainly free-living, predacious worms; (2) the Trematoda, or flukes, holozoic ecto- or endoparasites; and (3) the Cestoda, or tapeworms, saprozoic endoparasites in the enteron of vertebrates, whose larvae are found in the tissues of invertebrates or vertebrates.

Turbellaria are widespread in fresh water and the littoral zones of the sea, while one group of triclads occurs on land in moist habitats. Adult trematodes occur on, or in, practically all tissues and cavities of the vertebrates on which they feed. They are responsible for troublesome diseases in humans and animals. Larval flukes are frequent in mollusks, mainly gastropods, and occasionally occur in pelecypods. Vector hosts, such as insects and fish, are often interpolated between mollusk and vertebrate. Adult tapeworms, living in the enteron or the biliary ducts, compete with the host for food and accessory food factors such as vitamins. Larval tapeworms reside chiefly in arthropods, but larvae of one group, the Cyclophyllidea, develop in mammals, which may be severely impaired, or even killed, by the infection.

Bdelloura candida (Tricladida), ectocommensal on the king crab, Limulus

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

Platyhelminthes

[¦plad·ē·hel′min·thēz]

(invertebrate zoology)

A phylum of invertebrates composed of bilaterally symmetrical, nonsegmented, dorsoventrally flattened worms characterized by lack of coelom, anus, circulatory and respiratory systems, and skeleton.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Platyhelminthes

 

(flatworms), a large group of bilaterally symmetrical free-living or parasitic invertebrates. The Platyhelminthes are classified as a subphylum of the phylum Scolecida or as a separate phylum.

Flatworms range in length from 0.1 mm to several meters. The body is usually flattened and oval or somewhat elongate. It may be unsegmented or divided into several segments (tapeworms). Parasitic forms are equipped with such organs as suckers, proboscises, and hooks for attachment to a host. The body wall consists of a musculocutaneous sac, and the outer covering is a single layer of ciliated epithelium (in free-living forms) or a layer of basement membrane. The musculature is made up of annular, longitudinal, and transverse layers that enable the animal to move by means of a series of undulations. There is no body cavity, and the space between the musculocutaneous sac and the internal organs is filled with connective tissue, or parenchyma.

The mouth is situated ventrally or on the anterior end or, less commonly, the posterior end of the body. Most forms have a gut. In those flatworms lacking a gut (acoels), digestion takes place in the central part of the parenchyma. Tapeworms, which also lack a gut, feed by absorbing juices from the host’s intestine through their entire body surface. Flatworms do not have an anus.

The nervous system consists of a brain, lying at the anterior end of the body, and several paired longitudinal nerve cords, of which the lateral ones are usually the most developed. The sensory organs in the free-living forms include tactile papillae or tentacles, eyes, and organs of equilibrium (statocysts). There is no vascular system, and respiration is cutaneous. The excretory organs are protonephridia.

Flatworms are hermaphrodites with complex generative ducts. In most forms, the female portion consists of the ovary, which produces the ova, and the vitellarium, which supplies the ova with yolk and a shell. In free-living flatworms, development is direct or with metamorphosis. In the latter case, Miiller’s larvae are formed. A complex developmental cycle with the alternation of free-living and parasitic larvae and even of generations differing in structure is generally characteristic of parasitic flatworms. Free-living flatworms live in seas, in freshwaters, and on land. All others are external or internal parasites of animals and man.

Flatworms comprise four classes: Turbellaria, Monogenoidea, Trematoda, and Cestoidea.

REFERENCES

Rukovodstvo po zoologii, vol. 1. Moscow-Leningrad, 1937.
Beklemishev, V. N. Osnovy sramitel’noi anatomii bespozvonochnykh, 3rd ed, vols. 1–2. Moscow, 1964.
Doge!’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1974.

A. V. IVANOV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.