Flatworms

Flatworm Facts

Classification:

Kingdom: Animalia

Subkingdom: Eumetazoa

Phylum: Platyhelminthes

Classes: Turbellaria (free-living), Monogenea (ectoparasitic flukes with a one-host life cycle), Trematoda (endoparasitic flukes), Cestoda (tapeworms), (phylogenetic classification also includes Catenulida and Rhabditophora)

Subclasses: Cestoda—Cestodaria (body not segmented), Eucestoda (body segmented into proglottids), Aspidogastrea (no oral sucker)

Orders: Various (see below)

Geographical location: Worldwide

Habitat: Turbellaria—generally found in ponds, lakes, streams, and oceans; Monogenea, Trematoda, and Cestoda—larvae may be found in streams, but adults live within the body of a host

Gestational period: Varies among species, but most species lay eggs a few days after fertilization; eggs usually hatch a few days to a few weeks after being deposited

Life span: Varies among species; can be as short as a year in some turbellarians and up to thirty years in some flukes

Special anatomy: Elongated, bilateral invertebrates without appendages have neither a true body cavity nor a circulatory system; parasitic species have specially adapted mouth parts for attaching to the tissues of the host

Flatworms or Platyhelminthes are worm-like animals with a single major opening in the gut. This opening functions as both a mouth and an anus. Between the gastrodermis (lining of the gut) and the epidermis, the body is filled with tissues, including layers of muscle, connective tissue, and reproductive organs. The classification of flatworms is inconclusive in the literature, but several groups are proposed. Included in the flatworms are free-living forms, or those that do not depend on a host organism for food, (class Turbellaria) and two major groups of animal parasites (class Trematoda—flukes—and class Cestoda—tapeworms). Most of the 4,000 turbellarian species are bottom dwellers in marine water or freshwater or live in moist terrestrial environments, but a few species are symbiotic or parasitic. Most of the larger species are found on the underside of rocks or other hard objects in freshwater streams or the ocean's littoral zones.

All of the cestodes and trematodes exist as endoparasites, and most exhibit indirect life cycles with more than one host. The initial host is usually an invertebrate, and the final host is most often a vertebrate. A number of species utilize humans as the final host.

General Characteristics of Flatworms

Around 3,000 free-living flatworms in the class Turbellaria generally range in size from one to fifty millimeters. The epidermis is covered with cilia, and locomotion is achieved through a combination of ciliary movements and the contraction and relaxation of the layer of circular muscles that go around the body and a layer of longitudinal muscles that extend down the length of the body. The most commonly studied turbellarians are the planarians. The digestive system consists of a mouth on the ventral surface, a pharynx, and an intestine. Planarians are mostly carnivorous, feeding mainly on nematodes, rotifers, and insects. In contrast to the parasitic species, the turbellarians have a simple life cycle. Some can reproduce asexually by transverse fission, but most reproduce sexually. While the turbellarians are hermaphroditic, they generally crossbreed. Planarians are often used in studies about asexual reproduction because they demonstrate a remarkable ability to regenerate. If a section is excised from the middle of the worm, it will regenerate both a new tail and a new head.

The 11,000 species of flukes, or Termatodas, primarily differ from turbellarians in their adaptions for parasitism, including oral and ventral organs for adhesion, such as suckers or hooks, and an increased reproductive capacity. They are generally leaf-shaped, varying in size from ten to twenty millimeters. Most flukes, such as those in the Digenea class, have complex life cycles. The eggs the mature fluke produces pass from the definitive host and hatch in water to form free-swimming larvae called miracidia. The miracidium enters the tissue of an intermediate host, snails, and transforms into a sporocyst, reproducing asexually to form rediae. The rediae reproduce asexually to form cercariae, which leave the snail and penetrate a second intermediate host, such as fish, or encyst on vegetation, where they become metacercariae, juvenile flukes. When the definitive host eats the metacercariae, they develop into mature flukes. These flatworms have chemoreceptors and light receptors to locate their host. Mature flukes typically inhabit the circulatory system or liver of their host.

There are around 3,500 recognized species in the class of tapeworms, Cestoda. Like turbellarians, tapeworms are keenly adapted for parasitism, but unlike the flukes, they have long, slender bodies that can reach lengths of several meters and lack a digestive system. They obtain digested nutrients directly from the gut of the host. The tapeworm body consists of a linear series of proglottids. The tapeworm grows lengthwise by adding new proglottids. Mature proglottids contain fertilized eggs and break off the end of the tapeworm to be excreted out of the host. Almost all cestodes require at least two hosts, and the adult is a parasite in the digestive tract of vertebrates. One of the intermediate hosts is usually an invertebrate. Almost all species of vertebrates are subject to tapeworm infection.

Some scientists consider Monogenea a subclass of Trematoda, while others assert it is a class of flatworms. The number of species classified in this group is also debated, ranging from 1,100 to 7,000. These flatworms are primarily found on fish in warm or cold saltwater or freshwater and measure around 2 centimeters. Some monogeneans live on frogs, turtles, and salamanders, but the only known mammals to be infected are hippopotamuses, which may harbor the worms in their eyes. They are hermaphroditic but may reproduce sexually. They do not require an intermediate host to reach maturity and have only one host per lifetime.

Principal Terms

Definitive Host: the host in which a symbiont (the organism living within the host) matures and reproduces

Ectoparasite: a parasitic organism that attaches to the host on the exterior of the body

Endoparasite: a parasitic organism that attaches to an interior portion of the host’s body

Free-living: an organism that does not have to spend a portion of its life cycle attached to another organism

Hermaphroditic: a situation in which both functional ovaries and testes are present in the same organism

Proglottid: a body segment of a tapeworm that contains a set of reproductive organs, usually both ovaries and testes

Snailing: the process in which the free-swimming larva (miracidium) of the flukes utilizes the tissue of a snail as an intermediate host

Bibliography

du Preez, Louis H., et al. Polystomatid Flatworms State of Knowledge and Future Trends. New York City, Springer, 2024.

Ehlers, U. “Phylogenetic Relationships Within the Platyhelminthes.” In The Origins and Relationships of Lower Invertebrates, S. Morris, J. D. George, R. Gibson, and H. M. Platt, eds. Oxford, Clarendon Press, 1985.

“Flatworm (Turbellaria).” EcoSpark, 2022, www.ecospark.ca/flatworm. Accessed 30 June 2023.

Hickman, Cleveland P., et al. Integrated Principles of Zoology. 19th ed. New York City, McGraw-Hill, 2024.

Miller, Stephen, and Harley, John. Zoology 2016. 10th ed. New York City, Macmillan/McGraw-Hill, 2015.

Schell, Stewart C. Handbook of Trematodes of North America North of Mexico. Moscow, University Press of Idaho, 1985.

Schmidt, Gerald D. CRC Handbook of Tapeworm Identification. Boca Raton, CRC Press, 1986.

Strickland, G. Thomas. Hunter’s Tropical Medicine and Emerging Infectious Disease. 10th ed. Philadelphia, W. B. Saunders, 2020.