Flatworms (Platyhelminthes) room a group of bilaterally symmetrical, acoelomate, soft-bodied invertebrate pets found in marine, freshwater and moist terrestrial environments.

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Flatworm species include:

TurbellariaTapewormsPolycladidaHymenolepisGirardia tigrina



With an ext than 20,000 varieties currently identified, the phylum Platyhelminthes provides up one of the largest phyla after ~ chordates, mollusks, and arthropods. ~ above the various other hand, they make up the largest phylum among acoelomates. 


Kingdom: Animalia - Flatworms autumn under the animal kingdom the is identified by multicellular eukaryotic organisms. In some classifications, castle are additionally classified under the basal animal clade Eumetazoa since they room metazoans that loss under the animal kingdom (Kingdom Animalia).


Bilateria- Flatworms additionally fall under Bilateria in Eumetazoa. This group includes animals with bilateral the opposite consisting of a head and tail (as well as a dorsal part and belly).


Protostomia- as members that this clade, flatworms make up three germ layers. Together such, lock are likewise often referred to as protostomes.


Apart from these higher classifications, the phylum is further separated into the following classes:



Class Turbellaria

Turbellaria Flatworm - by brother Museum , via Wikimedia Commons

Class Turbellaria is composed of about 3,000 varieties of organisms spread throughout at the very least 10 orders. While a bulk of these species live in naval environments, over there are countless others that can be discovered in freshwater environments and also tropical terrestrial and also moist pleasant environments. As such, they need at least moist problems to survive. 

Depending top top the species, members of course Turbellaria one of two people exist together free-living organisms or parasites. For instance, members the the bespeak Temnocephalida exist together either entirely commensals or parasites. 

One the the finest examples the a free-living (opportunistic) varieties in this order is the Temnosewellia minor. In your environment, emnosewellia minor use their solid suckers to connect onto crayfish (at the tail section) from where they feed on miscellaneous smaller organisms prefer protozoa where the fish goes. As such, they carry out not reason harm come the crayfish and also only rely on the crayfish because that transport and stirring up atmospheres with food sources. 

Ichthyophaga subcutanea, top top the other hand, has actually been displayed to be a true parasite of such marine fish together Hexagrammus decagrammus. In together hosts, the parasite lives below the skin where it obtains the nutrition.

Other qualities of class Turbellaria


As worms, they have actually an elongated young that is fairly soft. The body is likewise have tapered at both end with decreased thickness contrasted to the central part of the body.With a compressed dorso-ventral body section, Turbellaria species have a high surface ar area come volume ratio.Whereas the marine species are very colorful (yellow spots, red in color etc.) those found in terrestrial and interstitial settings are mainly dull in appearance.They space not segmentedThey space acoelomate. As such, Turbellaria varieties lack a coelom (a human body cavity located between the body wall surface and intestinal canal in most animals).They have sub-epidermal rhabdites in your ciliated epidermis that identify this course from other flatworms.They lack an anal opening. Together a result, food product is take away in v the pharynx and also ejected with the mouth.While a majority of types in this class are predators of smaller sized invertebrates, rather live together herbivores, scavengers, and also ectoparasites.Pigment cells and also photoreceptors present in your eyespots is provided in place of image-forming eyes.Depending top top the species, the peripheral nervous system of Turbellarians arrays from very an easy to complicated interlacing nerve network that manage muscle movement.Reproduction is completed through a variety of methods that incorporate sexual reproduction (simultaneous hermaphrodites) and also asexual reproduction (transverse fission). In sex-related reproduction, eggs room produced and also bundled right into cocoons native which juveniles hatch and also develop. Asexually, some of the types split right into two halves that the regenerate to type the missing half thus occurring into a entirety organism.


Common assignment belonging to course Turbellaria include:


Order Acoela - about 600 species found in marine environmentsOrder Neorhabdocoela - about 600 varieties characterized by a saclike intestine, two longitudinal trunks, and also a worried systemOrder Macrostomida - 200 varieties often found between sand grains


Some the the other orders include:



Class Monogenea

By Patience C Igeh,Quinton M Dos Santos,Annemariè Avenant-Oldewage(2017).Redescription of Cichlidogyrus philander.CC through 4.0, https://commons.wikimedia.org/w/index.php?curid=66526945

This is just one of the largest groups of flatworms who members as virtually exclusively parasites of aquatic vertebrates (ectoparasites). 

Though they room grouped in a different course to trematodes, course Monogenea has been displayed to have many similar traits to trematodes. However, they have the right to be easily distinguished from trematodes and also cestodes by the fact that they possess a posterior organ recognized as a haptor. 

Monogeneans vary in size and also shape. For instance, conversely, larger varieties may appear flattened and shaped like a leaf (leaf-shaped) the smaller ones are more cylindrical. Rather than an oral sucker, monogeneans have actually a big posterior adhesive disk recognized as opisthaptor that renders it feasible to affix to the host. This structure consists of hooks that boost attachment.

Using these structures, monogeneans room well able to connect to such animal parts together fins, gills and the oral cavity from wherein they can feed off the outer epidermal great of the host.

The head that monogeneans in ~ the anterior region may save eyespots through pigments. This an ar of the body likewise has an oval shame pharynx the is poorly developed.

Characteristics of course Monogenea 

Some of the other characteristics of monogeneans include:


They are hermaphrodites - all members of class Monogenea space hermaphrodites. As such, they own both male and also female reproductive guts (testis and ovary) the are situated at the central part that the body. Fertilized egg are directly released in water whereby they flower to create free-swimming larva. To complete their life cycle, the larva infects a host as they build to proceed the cycle; Monogeneans perform not have actually intermediate master in your life cycle.They use adhesive organs known as haptors to connect to the host. This structure likewise consists that suckers, clamps or anchors.Although they have certain body shapes relying on the species, they have actually been presented to be capable of elongating/shortening their bodies as they relocate in their environmentThey do not have actually an anal opening and also therefore use protonephridial system because that the functions of excreting garbage productsThey lack a respiratory and circulatory system however have a nervous mechanism consisting that a nerve ring and also nerves that prolong to the posterior and also anterior component of the bodyAs parasites, monogeneans often feed on the skin cells mucus as well as the blood of the host which causes damage to the mucous membrane and also skin the protects the pet (fish). These infections can also influence other guts of the organize including the gill tissue

Class Cestoda

Class Cestoda is written of over 4,000 types commonly recognized as tapeworms. Typically, cestodes are internal worms (endoparasites) the require an ext than one host for their complex life cycles. 

Compared come the other varieties of flatworms, cestodes are identified by their long level bodies that can grow up to 18 meters long consisting of numerous reproductive systems (proglottid).

Class Cestoda is further separated into two various other subclasses the include:


Subclass Cestodaria - Members that this subclass (about 15 species) are unsegmented and can be uncovered in the intestine (or body cavity in some cases) the primitive fish. Cestodaria varieties are defined by a single set of the reproductive organ (either masculine or female), suckers, they absence a cradle system and also parenchymal muscle cells.

This subclass consists of the adhering to orders; Amphilinidea, Caryophyllidea, and also Gyrocotylidea.


Subclass Eucestoda - The majority of cestodes belong to this subclass and also are described as true tapeworms. The human body of this organisms is separated into scolex (containing suckers and/or hooks), a neck and a strobila that is composed of a collection of units recognized as proglottids. Because that members the this subclass, the proglottids play an important role in reproduction. 

Subclass Eucestoda is written of a number of orders the include: Tetraphyllidea, Lecanicephalidea, Diphyllidea, Trypanorhyncha, and Nippotaeniidea amongst a few others. With the subclass consists of end 3,000 species, varieties in the various orders are differentiated by the variety of suckers, visibility or absence of sex ducts as well as a framework of the scolex among other features.

Characteristics of course Cestoda

A complicated life cycle - The body of true tapeworms is composed of countless segments well-known as proglottids. Each of the proglottids includes both a male and female reproductive structures (as hermaphrodites) that are capable of reproducing independently.

Given that a solitary tapeworm can develop as numerous as a thousand proglottids; this enables tapeworms to proceed thriving. Because that instance, a single proglottid is qualified of creating thousands the eggs, their lifecycle can proceed in another host once the eggs are ingested.

Here, the organize that ingests the egg is well-known as the intermediate host given that that is in this certain host the the eggs hatch to develop a larvae (coracidium). The larvae, however, proceeds to develop in the second host (definitive host) and also mature in the adult stage.


They lack a cradle system - contrasted to the two various other classes that flatworms, tapeworms absence a cradle system. Instead, the surface ar of your bodies are covered by little microvillus-like projections similar to those uncovered in the little intestine of many vertebrates.

Through these structures, tapeworms efficiently absorb nutrients through their outer covering (tegument). For this reason, a bulk of tapeworms can be discovered in the tiny intestine of countless of your hosts wherein they can easily attain nutrition.

They have actually well-developed muscle.

Modified cilia top top their surface ar are supplied as sensory endings.

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The nervous system is made up of a pair of lateral nerve cords.


Examples the cestodes varieties include:


Taenia soliumDipylidium caninumTaenia saginataHymenolepis nanaDiphyllobothrium latum

Class Trematoda