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(invertebrate zoology)
A loose grouping of acoelomate, parasitic flatworms of the phylum Platyhelminthes; they exhibit cephalization, bilateral symmetry, and well-developed holdfast structures.



(also Digenea), a class consisting of flukes and other flatworms of the phylum Platyhelminthes. Adult forms parasitize vertebrates, including humans, while larvae parasitize mollusks and sometimes other invertebrates. The class embraces approximately 7,000 species, which are widely distributed. The body is usually elongate, flattened, and leaf-shaped; occasionally it is cylindrical or pear-shaped. Most species vary in length from a few tenths of a millimeter to several centimeters; Nemato-bothrium filaría, which parasitizes the gill cavity of the Mediterranean fish Sciella aquilla, measures as much as 1.3 m. The covering consists of an epithelium, usually with small spines, scales, or nodules. Typical trematodes have two suckers: an oral sucker at the anterior end of the body and a ventral sucker, most often in the middle portion of the body on the ventral side. The mouth opening is usually on the bottom of the anterior sucker, although in a number of forms it is in the ventral sucker. Some trematodes, for example, representatives of Echinostomatidae, have large chitinoid spines at the anterior end of the body. The suckers and spines serve to attach the parasite to the tissues of the host. The suckers are highly developed in forms that inhabit the lumen of the digestive tract, while in forms that parasitize ductless organs, they are underdeveloped or reduced. The coverings, together with underlying layers of musculature, form the musculocutaneous sac, which overlies the internal organs. There is no body cavity, the spaces between organs being filled with parenchyma. Nematodes have digestive, excretory, nervous, and reproductive systems, but no circulatory system or respiratory organs.

The digestive system begins with the mouth opening, followed by the pharynx, the esophagus, and usually a branched blind intestine; there is no anal opening, and waste materials are excreted through the mouth. In some trematodes, the posterior ends of the branches of the intestine open into the urinary bladder or to the exterior. Protonephridia serve as excretory organs. The nervous system consists of pairs of cerebral ganglia located near the pharynx, and six lateral paired bundles of nerve fibers that branch off from them. The reproductive system occupies most of the body. With a few exceptions, mainly the orders Schis-tosomatida and Didymozaidata, trematodes are hermaphroditic. Almost all are oviparous, although a few may be considered viviparous, since their larvae hatch from eggs while still in the uterus.

The miracidium, a ciliate larva, swims about in the water for a short time and implants itself in the body of a mollusk, the first intermediate host. In the body of the mollusk, the miracidium becomes a nonmotile pouchlike sporocyst, in which special cells form individuals of the next stage of development, the rediae. The rediae in turn produce cercariae, which are motile caudate larvae with a number of features characteristic of sexually mature trematodes. The sporocysts and rediae reproduce parthenogenet-ically and therefore are called parthenitae. The cercariae emerge from the mollusk into the external environment and in order to develop further must enter the body of a vertebrate. This vertebrate is considered the definitive host if there are two hosts in the trematode’s life cycle, or a second intermediate host if the life cycle includes three hosts. In the body of the second intermediate host, the cercaría is transformed into a metacercaria, which enters the definitive host and develops into an adult trematode. In a few species of Trematoda the developmental cycle is complex, for example, the sporocysts and rediae may yield daughter generations. In others it is simplified, and several stages of development are omitted. A reservoir host may become involved in the cycle.

Many species of Trematoda are known to be the causative agents of trematodiases. Measures for preventing infection of animals and humans require detailed knowledge of the parasites’ life cycles.

Trematodes are conventionally divided into three subclasses: Aspidogasterata, Gasterostomata, and Prosostomata. However, many taxonomists believe that Aspidogasterata should be an independent class. There is no unanimity in regard to the number of orders, which vary from three to 14. Phylogenetically, trematodes are considered to be related to the ancestors of the rhabdo-coelous class Turbellaria.


Skriabin, K. I. Trematody zhivotnykh i cheloveka: Osnovy trematodologii, vols. 1–25. Moscow, 1947–74.
Ginetsinskaia, T. A. Trematody, ikh zhiznennye tsikly, biologiia i evoliutsiia. Leningrad, 1968.
Yamaguti, S. Synopsis of Digenetic Trematodes of Vertebrates, vols. 1–2. Tokyo, 1971.