Phylum Platyhelminthes- characteristics, classification, examples

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Platyhelminthes (flatworms) definition

Platyhelminthes are triploblastic, bilaterally symmetrical, dorsoventrally flattened, acoelomate flatworms with organ grade of construction without a definite anus, circulatory, skeletal or respiratory system but with Protonephridial excretory system and mesenchyme filling the space between the various organ of the body.

Figure: Phylum Platyhelminthes. Source: WebMD and Wikipedia.

Phylum Platyhelminthes (flatworms) characteristics

They are free-living, commensal or parasitic.
They are bilaterally symmetrical and dorsoventrally flattened, triploblastic worm.
Bilaterally symmetrical with the definite polarity of head and tail ends.
Triploblastic i.e. body derived from three embryonic germ layers; ectoderm, mesoderm, and endoderm.
Dorsoventrally fattened i.e. well-developed ventral surface with mouth and gonopore.
Their body generally shapes as a worm but varies from moderately elongated flattened to long ribbon-like and leaf-like.
They are small to moderate in size varying from microscopic to extremely elongated form measuring up to 10-15 meters.
Their body is unsegmented except in class Cestoda.
The majority of them are white, colorless and some derive color from ingested food while free-living form are grey, brown-black or brilliantly colored.
Their anterior end of the body is differentiated into the head.
Mouth and genital pores on the ventral surface are well marked in turbellarians but less marked in cestodes and trematodes.
Their parasitic form has adhesive structures like hooks, spines and suckers, and adhesive secretions.
The body is covered with cellular or syncytial, frequently ciliated epidermis; while trematodes cestodes, lacks epidermis and their body covered with cuticle.
Exo- and endoskeleton are completely absent, hence the body is generally soft. The hard part consists of cuticle, spines, thorns, hooks, teeth.
They are acoelomate i.e. without any body cavity.
Space between various organs filled with special mesodermal tissues, the mesenchyme, and parenchyma.
Their digestive system is branched and incomplete without an anus and totally absent in acoela and cestode.
They lack skeletal, respiratory and circulatory systems.
The excretory system includes a lateral canal and a single or pair of protonephridia with flame cells or bulbs. Absent in some primitive form.
Their nervous system is primitive, ladder-like. The main nervous system consists of a pair of ganglia or brain and one or three pairs of longitudinal nerve cords connected by transverse nerves.
Their sense organs are simple. A common occurrence in tubellaria but greatly reduced in parasitic form. Chemo- and tangoreceptors commonly in the form of ciliated pits and grooves.
They are mostly monoecious (hermaphrodite).
Their reproductive system is highly evolved or complex in most of the forms.
Asexual reproduction occurs by fission in many freshwater turbellaria.
In the majority of form, eggs are devoid of yolk. They are produced separately in the yolk or vitelline glands.
Fertilization is internal but cross-fertilization in trematodes and self-fertilization in cestodes.
Their life cycle is complicated involves one or more hosts.
Parthenogenesis and polyembryony commonly occur trematodes and tapeworms.
Some tapeworm propagates by endogenous or exogenous budding.
The flatworm is either free-living or ecto-or endocommensals or parasitic.

Phylum Platyhelminthes (flatworms) Classification

The classification is from Hyman, L.H., (1951) up to suborder only with certain modifications.

Class 1- Turbellaria (L., turbella= a little string)

Mostly free-living but some ectocommensals and endocommensals or parasitic called
Terrestrial marine or freshwater.
Body unsegmented and covered with ciliated cellular or syncytial epidermis, containing mucus-secreting cells and rod-shaped body called
Mouth ventral. intestine preceded by the muscular pharynx.
Adhesive organs(suckers) abundantly present.
Sense organ i.e. Tango, chemo, and photoreceptors common in free-living forms.
The excretory system consists of protonephridia, the flame cells.
Mostly reproduction sexual, asexual and by regeneration.
Life cycle simple.

Order 1- Acoela

Small, exclusively marine, less than 2 mm.
Ventral mouth; no muscular pharynx and no intestine.
The excretory system is totally absent.
No flame cells, definite gonads, gonoducts, and yolk glands.
Mostly free-living, found under stones or bottom mud, algae, some live in the intestine of sea-urchins and sea-cucumbers.
Some colored or brown by symbiotic algae.
Examples: Convoluta, Ectocotyle, Afronta.

Order 2- Rhabdocoela

Small (less than 3mm) freshwater, marine, and terrestrial form.
Simple pharynx and sac-like intestine without diverticula.
Nervous system with 2 main longitudinal trunks.
Protonephridia excretory system.
Eye usually present.
The reproductive system comprises few compact gonads, gonoducts and a cuticularized structure instead of penis papilla present. Yolk gland present or absent.
Marine, freshwater or terrestrial. Free-living, commensal or parasitic form
Examples: Catenula, Microstomum, Macrostomum, Mesostoma.

Suborder 1. Notandropora

Exclusively freshwater forms.
Simple pharynx.
The excretory system consists of a single median protonephridia.
Testes single compact mass, penis unarmed.
No yolk gland.
Asexual fission occurs with the formation of the chain of zooids.
Examples: Catenula,

Suborder 2. Opisthandropora

Freshwater or marine form.
The excretory system consists of paired nephridia.
Testes compact, penis armed with a stylet.
No yolk gland.
Asexual reproduction with a chain of zooids.
Examples: Macrostomum, Microstomum.

Suborder 3. Lecithopora

Freshwater, marine or terrestrial forms.
Bulbose pharynx.
The excretory system consists of paired nephridia.
Separate ovaries and yolk glands.
Reproduction is exclusively sexual.
Mostly free-living, some commensals or parasitic form.
Examples: Anoplodium, Mesostoma.

Suborder 4. Temnocephalida

Freshwater ectocommensals form.
The anterior end of the body provided with 2-12 tentacles.
Posterior end of the body provided with 1-2 adhesive discs.
Dolii form pharynx.
Simple gonopore.
Examples: Temnocephala, Monodiscus.

Order 3- Alloecoela

Moderate-sized between 1 and 10mm.
Mostly marine, freshwater and brackish water form.
Pharynx simple, Bulbose or plicate; intestine straight or branched (short diverticula).
The excretory system consists of paired protonephridia having 2 or 3 main branches and nephridiopores.
Nervous system with 3or 4 pairs of longitudinal nerve cords provided with transverse connectives.
The reproductive system consists of numerous testes and a pair of ovaries.
Penis papilla is mostly present.
Some are ectoparasitic or ectocommensals in the habit.
Examples: Prorhynchus, Plagiostomum, Geocentrophora.

Suborder 1. Archophora

Marine form.
Plicate pharynx.
Primitive female reproductive system, no female ducts.
Male copulatory apparatus simple opening posteriorly.
Examples: Proporoplana (only examples).

Suborder 2. Lecithoepitheliata

Marine, freshwater or terrestrial form.
Simple or Bulbose pharynx.
Penis with the cuticular stylet.
Simple or none female ducts.
No yolk glands.
Nutritive cells surround ova.
Examples: Prorhynchus, Geocentophora.

Suborder 3. Cumulata

Freshwater or marine form.
Bulbose or plicate pharynx.
Intestine usually devoid of diverticula.
Unarmed penis.
The female reproductive system consists of germovitellaria or separate ovaries and yolk glands.
Examples: Hypotrichina.

Suborder 4. Seriata

Mostly marine and freshwater form.
Plicate pharynx.
Intestine usually with lateral diverticula.
The female reproductive system consists of separate ovaries and yolk glands.
Statocyst is mostly present.
Examples: Otoplana, Bothrioplana.

Order 4- Tricladida

Large-sized turbellarians (2 to 60cm long).
Marine, freshwater or terrestrial forms.
Mouth mid-ventral.
Pharynx plicate usually directed backward.
Intestine with 3 branches, each with many diverticula.
Eyes usually present.
Protonephridia as lateral networks with many nephridiopores.
The male reproductive system consists of 2 or numerous testes; a penis papilla present.
The female reproductive organ consists of a pair of ovaries with yolk glands and a copulatory brusa.
Single gonopore.
Examples: Gunda, Dugesia, Bdelloura, Geoplana.

Suborder 1. Maricola

Exclusively marine form.
A pair of eyes and auricular grooves present.
Typical penis papilla sometimes armed with the stylet.
Rounded copulatory brusa present.
Only sexual reproduction takes place.
Examples: Bdelloura,

Suborder 2. Paludicola

Mostly freshwater, rarely brackish water forms.
Eyes 2 to many or completely absent.
Brusa usually presents anterior to the penis.
Mostly asexual reproduction.
Examples: Planaria or Dugesia.

Suborder 3. Terricola

Terrestrial, tropical and subtropical forms.
Elongated body mostly.
2 to many eyes.
Brusa is mostly absent.
Male and female antra usually separate.
Asexual reproduction may also occur.
Examples: Bipalium, Geoplana.

Order 5- Polycladida

Moderate -sized turbellarians (2 to 20 mm).
Marine, many bottom dwellers or littoral zones.
Plicate pharynx, intestine highly branched.
The nervous system consists of numerous radially arranged nerve cords.
Numerous eyes.
Male and female gonopore separate.
No yolk glands.
Testes and ovaries are numerous and scattered.
Examples: Leptoplana, Notoplana, Cestoplana, Planocera, Thysanozoon.

Suborder 1. Acotylea

Usually vertical curtain-like pharynx.
Suckers absent behind the gonopore.
Nuchal type tentacles.
Eyes never occur as a pair of clusters on the anterior margin.
Examples: Euplana, Leptoplana, etc.

Suborder 2. Cotylea

Tubular pharynx.
Sucker present behind the female pore.
A pair of marginal tentacles bearing eyes or a cluster of eyes at the anterior margin.
Examples: Thysanozoon, Yungia.

Class 2- Trematoda (Gr., trematodes= having pore)

Ectoparasitic or endoparasitic commonly called
Body unsegmented dorsoventrally flattened leaf-like.
Teguments thick but without cilia and rhabdites.
Body undivided and covered with cuticle.
Suckers and sometimes hooks present.
Digestive tract incomplete consists of the anterior mouth, simple pharynx and two forked or many branches intestine; anus absent.
3 pairs of the longitudinal nerve cord.
Protonephridial excretory system consisting of flame cells.
Mostly hermaphrodites(monoecious).
Single ovary, 2 to many testes.
Development direct (in ectoparasites) or indirect (in endoparasites) with alternation of hosts.

Order 1. Monogenea

Mostly ectoparasites in cold-blooded aquatic vertebrates.
Oral suckers either weak or absent.
Anterior end provided with a pair of adhesive structures.
Posterior end provided with an adhesive disc usually with hooks.
Excretory pores paired situated anteriorly on the dorsal side.
Male and female gonopore usually separate.
Vagina one or two. Uterus is small with a few shelled eggs.
Only one host in the life cycle.
Free-swimming ciliated larva called
Examples: Diplozon, Polystoma, Gyrodactylus, Dactylogyrus.

Order 2. Digenea

Endoparasites of vertebrates and invertebrates.
2 suckers without hooks; oral sucker around the mouth and ventral sucker or acetabulum.
Single posterior excretory pore.
No vagina. The uterus usually long with many shelled eggs.
The life cycle complicated involving many larval stages.
One to more intermediate hosts in the life cycle.
Larval forms reproduce asexually before metamorphosis.
Examples: Fasciola, Bucephalus, Opisthorchis, Paragonimus, Schistosoma.

Order 3. Aspidocotylea (=Aspidogastraea)

No oral suckers.
Large ventral suckers subdivided into several suckers without hooks.
Only one testis in the male system.
Endoparasites in the gut of fishes and reptiles.
Examples: Aspidogaster, Cotylapsis, Stichocotyle.

Class 3- Cestoda (Gr., ketos, gridle+ eidos, form)

Endoparasitic in the intestine of vertebrates.
Commonly called tapeworm.
Body divided into many segmented (proglottids) but rarely undivided, elongated, flat, ribbon-like.
Tegument without microvilli.
Body without epidermis and cilia but covered with cuticle.
Anterior end (scolex) is provided with adhesive structures (hooks, suckers) except in cestodaria.
Mouth and digestive systems totally absent.
The excretory system consists of protonephridia with typical terminal flame
The nervous system usually comprises a pair of ganglia and 2 lateral longitudinal nerve cords.
Each mature segment or proglottids monoecious, with male and female organs.
Life cycle complicates usually involving 2 or more hosts.
Embryos with hooks.

Subclass 1. Cestodaria

Endoparasitic in the coelom or intestine of vertebrates.
Body unsegmented, leaf-like without scolex and strobila (monozoic).
No alimentary canal.
Only one set of the monoecious reproductive system.
Larva lycophore with 10 hooks.

Order 1. Amphilinidea

Endoparasitic forms in the coelom of fishes.
Body flattened, oval or elongated.
No sucker.
Scolex absent.
Protrusible pharynx.
Anterior end bears frontal glands.
Male and vaginal pores situated posteriorly.
The uterus is very much coiled opening near the anterior end.
Examples: Amphilina.

Order 2. Gyrocotylidea

Endoparasitic forms in the intestine of fishes.
Body elongated and flattened.
An anterior sucker and a posterior rosette-shaped adhesive organ present.
Anterior end bears eversible proboscis.
Uterine, male and vaginal pores are together situated in the anterior half of the body.
Uterus short straight runs directly to pores.
Examples: Gyrocotyle.

Subclass 2. Eucestoda

Endoparasitic form in the intestine of fishes.
Body long, ribbon-like.
The body is divided into scolex, neck, and strobila with many proglottids (polyzoic).
Scolex expanded bearing adhesive structures.
Mostly with several sets of monoecious reproductive organs.
Larva with 6 hooks.

Order 1. Tetraphyllidea

Endoparasitic forms; exclusively in the intestine of elasmobranch fishes.
Scolex with 4 leaf-like bothria (sessile suckers) often provided with
Testes are anterior to ovaries.
Vitelline glands scattered.
Cirrus armed with spines and hooks.
Common genital atrium marginal.
Examples: Phyllobothrium, Myzophyllobothrium.

Order 2. Diphyllidea

Parasitic in the intestine of elasmobranch fishes.
Scolex with 2 bothria and spiny head stalk.
Strobila consists of not more than 20 proglottids.
Examples: Echinobothrium.

Order 3. Trypanorhyncha

Parasitic in the spiral valve of the digestive tract of elasmobranch fishes.
Moderately sized body.
Scolex with 4 bothria and 4 protrusible spiny proboscides.
Vitellaria in cortical parenchyma placed in a continuous layer.
Testes extend behind the ovary posteriorly.
Lateral gonopores; ventrally open uterus.
Examples: Haplobothrium, Tetrarhynchus.

Order 4. Pseudophyllidea

Parasitic in the intestine of teleost fishes and terrestrial vertebrates.
Body segmented into strobila or unsegmented.
Scolex with 2 to 6 shallow bothria (Suckers) rarely without adhesive organs.
Bilobed ovary, testes numerous, follicular and scattered in the mesenchyma of proglottids.
Vitellaria follicular, numerous.
Midventral gonopores.
Examples: Bothriocephalus, Dibothriocephalus.

Order 5. Taenioidea or Cyclophyllidea

Parasitic in the intestine of reptiles, birds, and mammals.
Large-sized tapeworm.
Scolex bears 4 larges in cupped suckers (acetabula) often with an apical rostellum armed with hooks.
Ovary two or many lobed; uterine opening absent.
Gonopores on one or both margins.
The excretory system consists of 4 longitudinal vessels.
Vitellaria (yolk gland) single and compact.
Examples: Taenia, Echinococcus, Hymenolepis, Moniezia.

References

Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.