phylum Platyhelminthes
(the "flat-worms")


compiled by

R. A. Davis
Professor of Biology and Geology
College of Mount St. Joseph
Cincinnati, Ohio, 45233-1670


Taxonomic Framework
Generic Index
Material available for study
Directions for Completing the Laboratory Exercise
About this web-page -- Caveat lector !


The acoelomates are so called because each of them has neither a pseudocoel nor a true coelom.

The following are comments and other information relating to the acoelomates. This compilation definitely is neither comprehensive or exhaustive. Thus, in addition to the following, you should study as much other material related to the subjects mentioned as you "can get your hands on". To this end, toward the bottom of this WEB-page are listed some bibliographic references that might prove useful. There also is an ankyliography (a list of links) to web-sites that may contain some useful information.

If you have some suggestions as to how this WEB-page might be improved, please, contact me.

R. A. Davis
Professor of Biology and Geology
College of Mount St. Joseph
Cincinnati, Ohio, 45233-1670

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The following is based on the classificatory schemes presented in various publications, including Hickman, Roberts, and Larson (1997, 293 and 296). However, it may not include all taxa listed therein, because we may not have specimens of all of them for you to study. The various schemes of classification do not always correspond with one another, nor does that presented below correspond exactly (and sometimes not even inexactly) with other classificatory schemes. Moreover, there is no doubt that the scheme presented below will be subject to considerable change in future years.

(Please, recall that botanists tend to use the taxonomic level "Division" for what zo÷logists call "Phylum".)


kingdom Animalia (= Metazoa)

subkingdom Eumetazoa

phylum Platyhelminthes ----- the "flat-worms"

class Turbellaria

examples: Bdelloura, Dendrocoelopsis, Dugesia

class Trematoda ----- the "flukes"

examples: Clonorchis, Fasciola, Fasciolopsis, Gorgodera, Gorgoderina, Haematoloechus, Opisthorchis, Schistosoma

class Monogenea

examples: Gyrodactylus, Polystoma

class Cestoda ----- the "tapeworms"

examples: Dibothriocephalus, Diphyllobothrium, Dipylidium, Taenia

The following are other taxa of acoelomate animals. Some workers are convinced that they are related to the platyhelminths; others are not so convinced.

phylum Cycliophora

Some workers have concluded that these are pseudocoelomates rather than acoelomates.

phylum Gnathostomulida

phylum Nemertea (= Rhynchocoela)


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Listed below are some representative genera of some of the higher taxa.



higher taxa

Bdelloura Platyhelminthes, Turbellaria
Clonorchis Platyhelminthes, Trematoda
Dendrocoelopsis Platyhelminthes, Turbellaria
Dibothriocephalus Platyhelminthes, Cestoda
Diphyllobothrium Platyhelminthes, Cestoda
Dipylidium Platyhelminthes, Cestoda
Dugesia Platyhelminthes, Turbellaria
Fasciola Platyhelminthes, Trematoda
Fasciolopsis Platyhelminthes, Trematoda
Gorgodera Platyhelminthes, Trematoda
Gorgoderina Platyhelminthes, Trematoda
Gyrodactylus Platyhelminthes, Monogenea
Haematoloechus Platyhelminthes, Trematoda
Opisthorchis Platyhelminthes, Trematoda
Polystoma Platyhelminthes, Monogenea
Schistosoma Platyhelminthes, Trematoda
Taenia Platyhelminthes, Cestoda

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In order to understand this group of organisms, how they function, how they evolved, and so on, you will need to know a number of words, their definitions, and how and when to use the words. Thus, you should put together a list of such words, definitions, and notes. Here are some items to start you in compiling your own list.

Note that many terms in the material in this WEB-page have not been listed separately in this glossary. This does not mean that it would be a wise idea for you to ignore the terms in the material above.

(If a word you seek is not in the following, you might find it in the general glossary.)

cf.  =  compare to (from the Latin "conferre", "to compare")
q.v.  =  see the entry for the previous word (from the Latin "quod vide", "which see")

COELOM   (noun)

In certain animals there is a kind of body-cavity derived from mesoderm and which is lined with a tissue called peritoneum. It differs from a pesudocoel in that the latter is derived from the embryonic blastocoel. A true coelom commonly is called a "eucoelom" ("eu-" means "true").


The cavity within the blastula.

BLASTULA   (noun)

The "hollow-ball" stage in the embryonic development of certain animals. The cavity within the blastula is caled a "blastocoel".

EUCOELOM   (noun)

(See: "COELOM".)


In certain animals there is a kind of body-cavity derived from the embryonic blastocoel. It differs from a true coelom in that the latter is derived from mesoderm, rather than from the blastocoel; moreover, a true coelom is lined with a tissue called peritoneum.

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TBC  =  citation incomplete or otherwise suspect; 
it definitely needs to be checked against the original.
V  =  verified with the original publication
[ ]  =  note / annotation
{ }  =  source of information

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Abramoff, Peter, and Robert G. Thomson, 1994, Laboratory Outlines in Biology VI: W. H. Freeman and Company, New York, ix + 526 p., specifically p. 303-311.

Carolina Biological Supply Company, 1976, "Infected Snails": p. 6-7 IN Carolina Parasitology Cultures. Carolina Biological Supply Company, Burlington, North Carolina, 17 p.

Desowitz, Robert S., 1981, New Guinea Tapeworms & Jewish Grandmothers. Tales of Parasites and People: Avon Books [Hearst Corporation], New York, New York, 224 p. ----- V.

Hickman, Cleveland P., jr., and Lee B. Kats, 2004, Laboratory Studies in Integrated Principles of Zoology (12th edition): McGraw-Hill, Boston, xiv + 434 p. (spiral-bound; ISBN 0-07-243994-0), specifically, Exercise 9. "The Acoelomate Animals. Phylum Platyhelminthes", p. 135-154.

Hickman, Cleveland P., jr., Larry S. Roberts, and Allan Larson, 1997, Integrated Principles of Zoology (tenth edition): Wm. C. Brown, Dubuque, Iowa, xix + 901 p., specifically, chapter 15 "The Acoelomate Animals", p. 279-300.

Lytle, Charles F., 1996, General Zoology. Laboratory Guide (twelfth edition): Wm. C. Brown, Dubuque, Iowa, xvi + 368 p., especially Chapter 9, Platyhelminthes. p. 133-149.

Lytle, Charles F., and John R. Meyer, 2005, General Zoology Laboratory Side (fourteenth edition): McGraw-Hill, Boston, xxiii + 372 p. (ISBN 0-07-234900-X; spiral-bound), specifically, Chapter 9. "Platyhelminthes", p. 125-144.

Margulis, Lynn, and Karlene V. Schwartz, 1998, Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth: W. H. Freeman and Company, New York, xx + 520 p.

Purves, William K., David Sadava, Gordon H. Orians, and H. Craig Heller, 2001, Life. The science of biology (sixth edition): Sinauer Associates, Inc./W. H. Freeman and Company, specifically, Ch. 31 "Animal Origins and Lophotrochozoans".

Raven, Peter H., George B. Johnson, Jonathan B. Losos, and Susan R. Singer, "2005" (actually in print in 2004!), Biology (seventh edition): McGraw-Hill, Boston, xxiii + 1250 p., plus appendices, and so on. (ISBN 0-07-291845-4; hardbound).

Raven, Peter H., George B. Johnson, Susan R. Singer, and Jonathan B. Losos, 2002, Biology (sixth edition): McGraw-Hill, Boston, xxix + 1238 p., plus appendices, and so on. (ISBN 0-07-303120-8; hardbound), specifically p. 890-893.

Vodopich, Darrell S., and Randy Moore, 1999, Biology Laboratory Manual (fifth edition): WCB/McGraw-Hill, Boston, xi + 546 p., specifically, p. 367-373.

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(Editorial note: The word "ankyliography" is analogous to the word "bibliography", but is derived from the Greek word "ankylion", which denotes a link in a chain [as opposed to "biblion", "book"].)

The University of California Museum of Paleontology, Berkeley, California: Platyhelminthes web-page.

The University of California Museum of Paleontology, Berkeley, California: Nemertea web-page.

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Here is a list of preserved material of "acoelomates" (including microscope slides) that is available in the Department of Biology.

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A number of the laboratory-manuals listed in the Bibliography contain exercises you might find useful to guide your study, as do other publications not listed.

One of the best ways to stimulate your observational skills is by making drawings of the specimens you examine. Moreover, drawings definitely help jog the memory, once the specimens themselves no longer are available. Hence, you should make drawings of all the specimens you study. These do not need to be equal to the renderings of a professional artist. However, each of your drawings should be well labelled, both as to what is shown and as to the parts and features thereof. There should be a scale on each drawing, of course. Accompanying each drawing should be notes of your observations of colours, changes over the time you observed the specimen, behaviour of the animal (if it was alive), and so on. Again, the goal is to stimulate your observational skills and your memory.

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ABOUT THIS WEB-PAGE -- Caveat lector !

This web-page was constructed using MS-FrontPage and its version of html. The results have been previewed by way of MS Internet Explorer. Unfortunately, things that look fine when viewed with Internet Explorer sometimes look rather peculiar when viewed with some other WEB-browser. For example, there can be unexpected and unpredictable changes in the size, shape, style of type, or some combination of these. Moreover, sometimes different versions of Internet Explorer do not present the same web‑page in the same way; for example, hyphens in one version can mutate into empty rectangles in another. Alas!

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16 March 1999; 5, 9, and 15 February 2001; 9 February 2002; 15 January 2004; 1 February 2004; 22 and 29 January 2005; 04 February 2005; 18 February 2007; 16 March 2012