Platyhelminthes (pla-te-hel-MIN-thes) is made of two Greek roots that mean "flat worms" [flat -plato (πλάτω); and worm -helmis (ελμισ)].  The reference is to the flattened nature of the animals in this phylum.  The name was coined by Gegenbaur (1859).



The flatworms are acoelomate animals which comprise important parasites (flukes, monogeneans, and tapeworms) and the free-living trematodes.  According to Brusca and Brusca (2003) and Valentine (2004), much controversy has been generated as to the position of the flatworms in the phylogeny of animals. Brusca and Brusca (2003) illustrate the two current views. One is that the platyhelminths (a primitive turbellarian) either gave rise to the protostomes or are the sister group to the bilaterians (as indicated by Raff, 2001). This view is predicated on the apparent simple body plan of flatworms.  The alternative view is that an ancestral protostome gave rise to the platyhelminths by reduction and neoteny. Even the position of the platyhelminthes within the protostomes is in question. Valentine (2004) places them in the Eutrochozoa while Giribet et al. (2007) place them in the Platyzoa (=Parazoa).  Most recent texts place the Platyhelminthes at the base of the Protostomes or sisters to all other bilaterians (e.g. Brusca and Brusca 2003; Ruppert et al. 2004; and Pechenik 2005).  The flatworm problem is exacerbated by the lack of synapomorphies in the turbellarians and the structural simplifications as a consequence of the parasitic lifestyle in the non turbellarian taxa.

We accept the views of Giribet et al. (2007) and Edgecomb et al. (2011) that the Platyhelminthes occupy a clade within the protostomes (see Figure 1).  Furthermore, in explaining the simple body plans of the flatworms, we invoke the simplification of the mature animal in response to the parasitic condition.  Although the body plans are simple, the life histories and developmental stages of the parasitic taxa are quite complex. 

The acoel and nemertodermatid "turbellarians" are removed from the Platyhelminthes by the analyses of Giribet et al. (2007) and placed at the base of the bilaterians.  Thus, the Platyhelminthes, as defined by most sources, is polyphyletic.



1. The Flatworms

2. Turbellarian Clade

3. The Neodermata Clade

4. The Trematoda Clade

5. The Digeneid Fluke Clade

6. The Aspidogastrean Clade

7. The Cercomeromorpha Clade

8. The Monogeneid Fluke Clade

9. The Cestode Clade

Pl = Platyzoa

P = Protostomata

FIGURE 1. MAJOR CLADES OF THE PLATYHELMINTHES.  This cladogram roughly conforms to most taxonomic systems of the platyhelminthes.  Most of it came from Ruppert et al. (2004).  The overall topology of the cladogram is from Edgecombe et al. 2011.



The Turbellarian Clade (2)

The free-living flatworms comprise a large, and possibly, unrelated collection of taxa.  All of them have a ciliated epidermis, which small taxa use to swim and large taxa use to creep over the substrate.  The turbulence at the surface of the animal gives the group its name, turbellaria, which means whirlpool (Ruppert et al. 2004).  A gland of unknown function called the frontal gland is found in most turbellaria.  Their bodies are covered with other glands, most notable are the glands that produce rhabdites, small rods that form mucus when expelled to the outside.  The most obvious character is the paired simple ocelli (also called eyespots) at the anterior end of the animal; however, they do have a covering of other sensory receptors.    Most taxa have filled the body cavity with a cellular parenchyma, but the catenulids, the most basal of the turbellarians, have a pseudocoelom that functions as a hemocoel (Ruppert et al. 2004).  All have a feeding mouth that usually is located near mid body and is at the end of a  eversible pharynx.  The digestive system is a blind caecum, sometimes highly branched in the larger taxa.

The diversity of the turbellarians is astounding and includes at least 8 different groups. Though most turbellarians are quite small and resemble ciliates, the animals that come to mind when considering free-living flatworms are planarians (actually Dugesia, Figure 2).  They are relatively large flatworms that are known for their cross-eyed appearance and a head that looks like a yield sign because of the laterally-projecting auricles on its head.  The animals are famous for being able to regenerate missing parts of the body, even forming a two-headed animal if cut appropriately.  When I was a student, several times I collected wild Dugesia for a group of experimental psychologists who called themselves the Worm Runners.  Most of their experiments involved training the flatworms to run a simple maze and then testing them for changes in RNA, etc.  They even tried to transmit memory from one worm to the next by grinding up a trained worm and feeding it to a naive one.  

Not all turbellarians are small or nondescript.  Many are brightly colored and occur in marine environments, freshwater, rainforests, and even the interstices of sand.  Some are quite large and showy (Figure 3).  An endemic turbellarian from Lake Baikal is more than a meter long (Ruppert et al. 2004)



The Neodermata Clade (3)

The Neodermata are defined by the synapomorphy of a syncytial epidermis, called a neodermis or cuticle, in the adult (Walker and Anderson (2001).  This clade includes the parasitic flatworms, most of which utilize more than one host in the life cycle.


The Trematoda Clade (4)

Members of this clade have a larval form that has bands of ciliated epithelium which alternate with syncytial neoderm.  Mollusks are the primary hosts.

The Digeneid Fluke Clade (5)

The digeneids are leaf-shaped animals with a sucker at the anterior end and one on the ventral side.  It has a pharynx and a digestive tract.  flukes that require two hosts: a molluscan intermediate host and a vertebrate terminal host in which sexual reproduction occurs.  The life history includes three larval forms before the adult develops in the vertebrate.  In general, eggs are produced by the sexually mature animals in the vertebrates.  The eggs pass out with the feces and, if they land in water, germinate to produce a miracidium, a small swimming larva with bands of cilia.  The miracidium finds and infects an appropriate molluscan in which it replicates by asexual reproduction, all of which  develop into into rediae.  The rediae in turn replicate asexually and ultimately form cercariae.  A cercaria is a swimming larval form that may seek out a vertebrate host directly, encapsulate on vegetation, or in another intermediate host where they might be consumed by the vertebrate host.  There are many different species of flukes, some of which infect the liver (Sheep Liver Fluke), the circulatory system (The Blood Fluke), lungs (e.g. Probolitrima, Figure 4), etc.

The Sheep Liver Fluke (Fasciola hepatica, Figure 5) is, as the name says, a parasite of sheep and their relatives, but the fluke has jumped to humans as an appropriate alternate host.  The mature adult lives in the liver and expels its eggs into the intestine via the gall bladder.  The eggs (Figure 6-a), upon reaching water, hatch into a motile miracidium (Figure 6-b), which then seeks out an appropriate snail (e.g. Fossaria modicella) and burrows into its body.  Once inside, the miracidium transforms into a sprorocyst that reproduces asexually to make rediae, which also reproduce asexually to generate cercariae (Figure 6-c).  The cercariae, which look like small tadpoles, leave the snail and swim until they come into contact with an appropriate aquatic plant (e.g. watercress).  There, the cercaria attaches, encapsulates itself, and develops into the infective metacercaria (Figure 6-d).  Once eaten by a sheep or a human, the metacercaria emerges from its capsule while in the duodenum; it burrows through the intestinal wall, migrates through the coelom, and burrows into the liver.  In the liver, the flukes feed on liver tissue and make lesions that can be debilitating. (See the attached Life Cycle of Fasciola hepatica). 

The Blood Fluke (Schistosoma mansoni, Figure 7) has a slightly shorter life cycle.  The mature worms live in the circulatory system; however, the sexes are separate, and they are very sexually dimorphic.  The male is relatively large with a large ventral groove in which the much smaller and thinner female resides.  When the female is gravid, the pair make their way to the capillary beds surrounding the large intestine, and lay their eggs.  The blockage caused by the eggs (Figure 8-a) and the actions of the adults cause an ulceration to develop and the eggs end up in the feces.  As with Fasciola, the eggs hatch in contact with water and the miracidium seeks out an appropriate host snail.  There, the development continues as it did in Fasciola.  However, the emerging cercaria (Figure 8-b) has a forked tail, and it swims directly to an appropriate host.  There, it burrows into the skin, loses its tail and begins to develop in the circulatory system. (See the attached Life Cycle of Schistosoma japonicum).

The Aspidogastrea Fluke Clade (6)

This small group of flukes is similar to the digeneids.  They also are similar with regard to details of their life histories in which they make ciliated larvae and alternate between a mollusk and a vertebrate, usually a fish or a turtle.  However, there seems to be lower host specificity than in the digeneids.   The mature worms have a very large sucker that usually is divided to form a series of suckers which cover the ventral part of the animal.

Aspidogaster (Figure 9) has a simple life history in which the fluke lives out its life in the pericardial cavity of a clam.  However, when the clam is eaten by a turtle, Aspidogaster can persist there in its gut.  Perhaps, it can even reproduce there.  (See the Life Cycle of Aspidogaster).







The Cercomorpha Clade (7)

The Cercomeromorphae (most of the Monogenians and Cestodes) are defined by the occurrence of a cercomere, a posterior hook-bearing structure on the larva.  

The Monogeneid Fluke Clade (8)

The monogeneids look like the trematodes (e.g. Figure 10), but they have no intermediate host.  Mostly they are ectoparasites on fish. The eggs hatch into onchomiricidia larvae which attach to a host and develop into an adult. Thus, one adult comes from each egg (rather than the potential hundreds of the trematodes).  The body has  paired excretory pores and a well-developed gut.  The two subclasses are separated by the type of posterior attachment organ (the opisthaptor).  Most organs of attachment are posterior and large.  In fact few taxa of the monogeneids have retained the anterior sucker.  (See the Life Cycle of Dactylogyrus and Figure 11).

The Cestode Clade (9)

The tapeworms are the most derived of the parasitic flatworms.  Adults inhabit the digestive tracts of vertebrates and the larval stages utilize a variety of intermediate hosts. The body has neither a gut nor a mouth and food is absorbed directly through the neodermis. The anterior end (actually an anterior zoid) usually has a rostellum and a scolex followed by a short neck and strobila, a a linear series of proglottids. Each proglottid is the product of budding, and is a reduced adult that develops a set of reproductive organs and matures by filling with eggs. The eggs develop into onchosphere larvae with 6-10 hooks.

The life histories of some of the tapeworms are fairly simple.  The Pig Tapeworm (Taenia solium) matures in humans and in pigs, too (Figure 12).  However, a mature proglottid which is released in the feces can shed many eggs which are viable.  The pig swallows the egg, which emerges as an onchosphere in the gut and burrows into the circulatory system.  They lodge in parts of the body, particularly in the muscles, where they encapsulate and develop into cysticerci.  If a person eats poorly cooked infected pork, the cysticerci emerge, attach to the mucosa with the anterior end, and develop into a mature tapeworm.  If a person swallows eggs, however, the onchospheres migrate all over the body and can lodge in the brain, heart muscle or other vital tissues bringing on rapid decline and death.  (See the Life Cycle of Taenia solium).

The Fish Tapeworm (Diphyllobothrium latum) becomes sexually mature in bears, dogs and other fish-eating mammals, humans included.  At maturity, the Fish Tapeworm can stretch to 10m!  The scolex does not have hooks; so, it anchors itself by lodging the scolex in the mucosa or even by penetrating it.  The lifecycle is somewhat more elaborate.  The mature egg hatches into a miracidium which is taken up by a copepod.  The miracidium matures into a procercoid in the hemolymph of the crustacean.  The procercoid infects a fish after the copepod is consumed.  The procercoid then burrows through the intestine and into the muscle tissue of the fish where it becomes a plerocercoid.  It is the plerocercoid that is infective to the mammal.

The last cestode strategy that I will present is that of Echinococcus granulosus (Figure 15), a small tapeworm of mammalian Carnivora.  As a mature animal, Echinococcus rarely has more than 3 proglottids.  A dog can be infected with hundreds and show no symptoms.  The eggs pass with the feces of the carnivore and attach to vegetation which can be eaten by quite a range of herbivores.  In the herbivore, the egg hatches, the miracidium burrows through the mucosa wall, encapsulates and begins to reproduce asexually.  The hydatid cyst can be very large (on the order of liters) and contain thousands of infective hydatid larvae.  The cysts can develop anywhere in the body, including the brain and body cavity.  A herbivore thus infected will be slow and easy prey for a wolf or coyote (or dog).  Each hydatid swallowed has the potential to develop into one of the diminutive tapeworms.  Humans can be mistaken for a herbivore by a miracidium and hydatid cysts can develop in people as in cattle. (See the Life Cycle of Echinococcus granulosus).








Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia.

Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257. 

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago.

Conway Morris, S. and J. S. Peel. 2008. The earliest annelids: Lower Cambrian polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland. Acta Palaeontol. Pol. 53(1): 137-148.

Darwin, C. R. 1881. The Formation of Vegetable Mould, Through the Action of Worms, With Observations on their Habits. John Murray. London.

Edgecombe, G. D., G. Giribet, C. W. Dunn, A. Hejnol,R. M. Kristensen, R. C. Neves, G. W. Rouse, K. Worsaae, and M. V. Sorensen. 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms Diversity and Evolution.  DOI 10.1007/s13127-011-0044-4. 

Frelich, L., C. Hale, S. Scheu, A. Holdsworth, L. Heneghan, P. Bohlen, and P. Reich. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions. 8(6): 1235-1245. 

Gegenbaur, C. 1859. Gundriss der vergleichenden Anatomie. Leipsig, Germany.

Giribet, G., C. W. Dunn, G. D. Edgecombe, and G. W. Rouse. 2007. A modern look at the Animal Tree of Life.  Zootaxa. 1668: 61-79.

Giribet, G., A., A. Okusu, A. R. Lindgren, S. W. Huff, M. Schrodl, and M. K. Nishiguchi. 2006. Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons. Proc. Nat. Acad. Sci. USA. 103(20): 7723-7728.

Halanych, K. M. 2004. The new view of animal phylogeny.  Annu. Rev. Ecol. Evol. Syst. 35: 229-256.

Halanych, K. M., T. G. Dahlgren, and D. McHugh. 2002. Unsegmented annelids? Possible origins of four lophotrochozoan worm taxa. Integ. and Comp. Biol. 42: 678-684.

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis .

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York. 

McHugh, D. 1997. Molecular evidence that echiurans and pogonophorans are derived annelids. Proc. Nat. Acad. Sci. USA. 94: 8006-8009.

Meglitsch, P. A. and F. R. Schramm. 1991. Invertebrate Zoology. Oxford University Press, New York, Oxford.

Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. 

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. 

Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963.

Siddall, M. E., E. Borda, and G. W. Rouse. 2004. Toward a tree of life for Annelida.  In: Cracraft, J. and M. J. Donoghue, eds. Assembling the Tree of Life. Oxford University Press. Oxford, New York.  pp. 237-251.

Sigwart, J. D. and M. D. Sutton. 2007.  Deep molluscan phylogeny: synthesis of palaeontological and neontological data.  Proc. Royal Society B. 274: 2413-2419..

Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York.

Struck, T. H., N. Schult, T. Kusen, E. Hickman, C. Bleidorn, D. McHugh, and K. M. Halanych. 2007. Annelid phylogeny and the status of Sipuncula and Echiura.  BMC Evolutionary Biology. 7:57  doi: 10.1186/1471-2148-7-57 

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

Walker, J. C. and D. T. Anderson. 2001. The Platyhelminthes, Nemertea, Entoprocta, and Gnathostomulida. In: Anderson, D.T., ed. Invertebrate Zoology. Oxford University Press. Oxford, UK. pp. 59-85. [L]

Valentine, J. W. 2004. The Origin of Phyla. University of Chicago Press. Chicago.  614 pp.

Zrzavý, J., P. Ríha, L. Piálek, and J. Janouskovec. 2009. Phylogeny of Annelida (Lophotrochozoa): total-evidence analysis of morphology and six genes. BMC Evolutionary Biology. 9:189  doi: 10.1186/1471-2148-9-189 


By Jack R. Holt.  Last revised: 02/01/2014