In this article we will discuss about Platyhelminthes:- 1. General Characters of Platyhelminthes 2. Classification of Platyhelminthes.
General Characters of Platyhelminthes:
1. Platyhelminthes are bilaterally symmetrical and dorsoventrally flattened, triploblastic worms.
2. Body shape generally worm-like but varies from moderately elongated flattened shape to long flat ribbons and leaf-like.
3. The flatworms are small to moderate in size varying from microscopic to extremely elongated forms measuring up to the 10-15 metres.
4. Majority of flatworms are white, colourless, some derive colour from the ingested food, while the free-living forms are brown, grey, black or brilliantly coloured.
5. Anterior end of the body is differentiated into the so called head.
6. Ventral surface bearing mouth and genital pores is well marked in turbellarians but is less marked in trematodes and cestodes.
7. Presence of great variety of adhesive secretions, organs of attachment and adhesion (e.g., hooks and suckers).
8. Body is covered with a cellular syncytial one layered partly ciliated epidermis; while in parasitic trematodes and cestodes, epidermis is lacking and the body is covered with cuticle.
9. Exo- and endoskeleton are completely absent, hence, the body is generally soft. Hard parts consist of cuticle, spines, thorns, hooks, teeth, etc.
10. Acoelomate, i.e., true coelom is absent.
11. Body space between the various organs is filled with a mesenchyme usually called parenchyma.
12. Digestive system is totally absent in Acoela and tapeworms but in other flatworms it consists of mouth, pharynx and blind intestine (anus absent),
13. Respiratory and circulatory systems are absent.
14. Excretory system consists of single or paired protonephridia with flame cells or bulbs. In Acoela the protonephridia are absent.
15. Nervous system is primitive. The main nervous system consists of a pair of cerebral ganglia or brain and one to three pairs of longitudinal nerve cords connected to each other by transverse commissures. This type of nervous system is called ladder type of nervous system.
16. Sense organs are of common occurrence in Tubellaria but these are greatly reduced in parasitic forms. Chemo-and tango receptors commonly occur in the form of ciliated pits and grooves.
17. Sexes are united, i.e., hermaphrodite with very few exceptions.
18. Reproductive system is highly evolved or complex in most of the forms.
19. Asexual reproduction by fission occurs in many freshwater Turbellaria.
20. In majority of forms, eggs are devoid of yolk but provided with special yolk cells and are covered by egg shell.
21. Cross-fertilisation in trematodes and self-fertilisation in cestodes is very common. Fertilisation is internal.
22. Life cycle complicated involves one or more hosts.
23. Parthenogenesis and polyembryony commonly occur in trematodes and tapeworms. Some tapeworms propagate by endogenous or exogenous budding.
24. The flatworms are either free-living or ecto- or endo-commensals or parasites.
2. Classification of Platyhelminthes:
The classification adopted here is from Hyman, L.H., (1951) up to suborders only with certain modifications.
Class 1. Turbellaria (L., turbella = a little string):
1. Mostly free-living flatworms but some ectocommensals and endocommensals or parasitic.
2. Body un-segmented.
3. Body covered with a cellular or syncytial epidermis usually with mucous cells and which is usually partly ciliated.
4. Adhesive organs abundantly present.
5. Digestive system usually consists of mouth, pharynx and intestine, anus not found.
6. Excretory system consists of protonephridia, the flame cells.
7. Sense organs consist of tango receptors and chemoreceptors.
8. Mostly hermaphrodite.
9. Reproduction sexual, asexual and by regeneration.
10. Life cycle simple.
Order 1. Acoeia:
1. Small turbellarians exclusively marine.
2. Digestive system consists of ventral mouth and simple pharynx.
3. Intestine absent.
4. Excretory system totally absent.
5. Definite gonads, gonoducts and yolk glands absent.
6. Usually free-living found under stones or on bottom mud; some live in the intestine of sea-urchins and sea-cucumbers.
7. Some coloured or brown by symbiotic algae.
Examples:
Convoluta, Ectocotyle, Afronta.
Order 2. Rhabdocoela:
1. Small freshwater, marine and terrestial forms.
2. Digestive tract complete with simple pharynx and intestine sac-like without diverticula.
3. Nervous system with two main longitudinal trunks.
4. Eye usually present.
5. Excretory system consists of protonephridia.
6. Reproductive system comprises few compact gonads, gonoducts and a cuticularised structure instead of penis papilla present.
7. Free-living, commensal or parasitic forms.
Suborder 1. Notandropora:
1. Exclusively freshwater forms.
2. Pharynx simple.
3. Excretory system consists of single median protonephridia.
4. Testis single compact mass, penis unarmed.
5. Yolk glands absent.
6. Asexual fission occurs with formation of chains of zooids.
Examples:
Catenula, Rhycoscolex.
Suborder 2. Opisthandropora:
1. Freshwater or marine forms.
2. Excretory system consists of paired nephridia.
3. Testes compact, penis armed with a stylet.
4. Yolk glands absent.
5. Asexual reproduction with chains of zooids.
Examples:
Macrostomum, Microstomum.
Suborder 3. Lecithopora:
1. Freshwater, marine or terrestrial forms.
2. Pharynx bulbose.
3. Excretory system consists of paired nephridia.
4. Separate ovaries and yolk glands.
5. Reproduction exclusively sexual.
6. Mostly free-living, some commensals or parasitic forms.
Examples:
Anoplodium, Mesostoma.
Suborder 4. Temnocephalida:
1. Freshwater ectocommensal forms.
2 Anterior end of body provided with 2-12 tentacles.
3. Posterior end of the body with one or two adhesive discs..
4. Pharynx dolii form.
5. Gonopore simple.
Examples:
Temnocephala, Monodiscus.
Order 3. Alloeeoela:
1. Moderate-sized turbellarians, mostly marine, few freshwater and brakish water forms.
2. Pharynx simple, bulbose or plicate; intestine with short diverticula.
3. Excretory system consists of paired protonephridia having two or three main branches and nephridiopores.
4. Nervous system with three or four pairs of longitudinal nerve cords provided with transverse connectives.
5. Reproductive system consists of numerous small testes and a pair of ovaries.
6. Penis papilla usually present.
7. Some are ectoparasitic or ectocommensals in habit.
Suborder 1. Archophora:
1. Marine form.
2. Pharynx plicate.
3. Female reproductive system primitive, no female ducts.
4. Male copulatory apparatus simple opening posteriorly.
Example:
Proporoplana (only example).
Suborder 2. Lecithoepitheliata:
1. Marine, freshwater or terrestrial forms.
2. Pharynx simple or bulbose.
3. Penis with cuticular stylet.
4. Female ducts simple or none.
5. Yolk glands absent.
6. Nutritive cells surrounds ova.
Examples:
Prorhynchus, Geocentophora.
Suborder 3. Cumulata:
1. Freshwater or marine forms.
2. Pharynx bulbose or plicate.
3. Intestine usually devoid of diverticula.
4. Penis unarmed.
5. Female reproductive system consists of germovitellaria or separate ovaries and yolk glands.
Example:
Hypotrichina.
Suborder 4. Seriata:
1. Mostly marine and few freshwater forms.
2. Pharynx plicate.
3. Intestine usually with lateral diverticula.
4. Female reproductive system consists of separate ovaries and yolk glands.
5. Statocyst mostly present.
Examples:
Otoplana, Bothrioplana.
Order 4. Tricladida:
1. Large sized turbellarians, marine, freshwater or terrestrial forms.
2. Pharynx plicate usually directed backwards.
3. Intestine with three highly diverticulated branches.
4. Eyes usually present.
5. Protonephridial excretory system present as lateral network with many nephridiopores.
6. Male reproductive organs consist of two to numerous testes; a penis papilla present.
7. Female reproductive organs consist of a pair of ovaries with yolk glands and a copulatory bursa.
8. Gonopore single.
Suborder 1. Maricola:
1. Exclusively marine forms.
2. A pair of eyes and auricular grooves present.
3. Typical penis papilla sometimes armed with stylet.
4. Rounded copulatory bursa is generally present.
5. Only sexual reproduction takes place.
Examples:
Bdelloura, Ectoplana.
Suborder 2. Paludicola:
1. Mostly freshwater, rarely brackish water forms.
2. Eyes two to many or completely absent.
3. Bursa usually present anterior to penis.
4. Reproduction mostly asexual.
Example:
Planaria or Dugesia.
Suborder 3. Terricola:
1. Terrestrial, tropical and subtropical forms.
2. Body mostly elongated.
3. Eyes two to many.
4. Bursa mostly absent.
5. Male and female antra usually separate.
6. Asexual reproduction may also occur.
Examples:
Bipalium, Geoplana.
Order 5. Polycladida:
1. Moderate-sized turbellarians, exclusively marine.
2. Body usually broad and flattened, some are elongated.
3. Pharynx plicate opens into main intestine.
4. Intestine with numerous branches and highly diverticulated.
5. Nervous system consists of numerous radially arranged nerve cords.
6. Eyes numerous.
7. Testes and ovaries are numerous and scattered.
8. Yolk glands absent.
9. Gonopores separate.
Suborder 1. Acotylea:
1. Pharynx usually vertical curtain-like.
2. Sucker absent behind the gonopore.
3. Tentacles nuchal type.
4. Eyes never occur as a pair of clusters on the anterior margin.
Example:
Notoplana.
Suborder 2. Cotylea:
1. Pharynx tubular.
2. Sucker present behind the female pore.
3. 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 pores):
1. Ectoparasitic or endoparasitic forms, commonly called flukes.
2. Body shape usually leaf-like, dorsoventrally flattened.
3. Body wall without epidermis and cilia.
4. Body undivided and covered with a cuticle.
5. Well developed suckers usually present.
6. Digestive tract incomplete consisting of mouth, pharynx and two forked or many branched intestine; anus absent.
7. Protonephridial excretory system consisting of flame cells.
8. Mostly hermaphrodite (monoecious).
9. Ovary single, testes two to many.
10. Life history simple or complicated.
Order 1. Monogenia:
1. Ecto-or endoparasitic forms of vertebrates.
2. Oral sucker either weak or absent.
3. Anterior end provided with a pair of adhesive structures.
4. Posterior end provided with an adhesive disc usually with hooks.
5. Excretory pores paired situated anteriorly on the dorsal side.
6. Male and female gonopores usually separate.
7. Free-swimming ciliated larva called onchomiracidium.
8. Only one host in life cycle.
Examples:
Diplozoon, Polystoma, Gryodactylus.
Order 2. Aspidobothria:
1. Endoparasites in the gut of fishes and reptiles.
2. Oral sucker absent.
3. Anterior end without paired adhesive structures.
4. The entire ventral surface acts as adhesive organ but subdivided into compartments without hooks.
5. Nephridiopore single.
6. Only one testis in male reproductive system.
7. Life cycle simple, no alternation of hosts.
Examples:
Cotylapsis, Aspidogaster.
Order 3. Digenia:
1. Endoparasites of vertebrates and invertebrates.
2. Mostly with two suckers without hooks; an oral sucker around the mouth and the ventral sucker or acetabulum.
3. Excretory pore single at the posterior side.
4. Vagina absent, uterus usually long with many shelled eggs.
5. Life cycle complicated involving many larval stages.
6. One to more intermediate hosts in life cycle.
Examples:
Fasciola, Schistosoma,
Bucephalus, Opisthorchis (=Clanorchis).
Class 3. Cestoda (Gr., kestos = girdle + eidos = form):
1. Endoparasites in the intestine of vertebrates.
2. Commonly called tapeworms.
3. Body without epidermis and cilia but covered with cuticle.
4. Body usually divided into few to many segments (proglottids), rarely undivided.
5. Anterior end (scolex) is provided with adhesive structures (hooks, suckers) except in Cestodaria.
6. Mouth and digestive tract totally absent.
7. Excretory system consists of protonephridia with typical terminal flame bulbs.
8. Nervous system usually comprises a pair of ganglia and two lateral longitudinal nerve cords.
9. Each segment (proglottis) contains one or two sets of complete hermaphroditic reproductive system.
10. Life cycle complicated usually involving two or more hosts.
11. Embryos possess hooks.
Subclass 1. Cestodaria:
1. Endoparasitic in coelom or intestine of vertebrates.
2. Body undivided (monozoic).
3. Scolex absent.
4. Alimentary canal absent.
5. Only one set of monoecious reproductive organs.
6. Larva lycophore with ten hooks.
Order 1. Amphilinidea:
1. Endoparasitic forms in the coelom of fishes.
2. Body flattened, oval or elongated.
3. Scolex absent.
4. Suckers absent.
5. Anterior end bears protrusible proboscis and frontal glands.
6. Male and vaginal pores situated posteriorly.
7. Uterus is very much coiled opening near the anterior end.
Example:
Amphilina.
Order 2. Gyrocotylidea:
1. Endoparasitic forms in the intestine of fishes.
2. Body elongated and flattened.
3. An anterior sucker and a posterior rosette- shaped adhesive organ present.
4. Anterior end bears eversible proboscis.
5. Uterine, male and vaginal pores are together situated in the anterior half of the body.
6. Uterus short, straight runs directly to pore.
Example:
Gyrocotyle.
Subclass 2. Eucestoda:
1. Endoparasitic forms in the intestine of fishes.
2. Body usually very elongated ribbon-like.
3. Body divisible into anterior scolex, neck and strobila consisting of few to many proglottids (polyzoic).
4. Scolex expanded bearing adhesive structures.
5. Each proglottis with more than one set of monoecious reproductive organs.
6. Larva with six hooks.
Order 1. Tetraphyllidea:
1. Endoparasitic forms; exclusively in the intestine of elasmobranch fishes.
2. Scolex bears four bothridia (sessile suckers) often provided with hooks.
3. Testes lie in front of ovaries.
4. Vitelline glands scattered in two lateral bands.
5. Cirrus armed with spines or hooks.
6. Common genital atrium marginal.
Examples:
Phyllobothrium, Myzophyllo- bothrium.
Order 2. Diphyllidea:
1. Parasitic in the intestine of elasmobranch fishes.
2. Anterior scolex with two bothria and a spiny head stalk.
3. Strobila consists of not more than 20 proglottids.
Example:
Echinobothrium.
Order 3. Trypanorhyncha:
1. Parasitic in the spiral valve of digestive tract of elasmobranch fishes.
2. Body size moderate.
3. Scolex provided with four bothria and four protrusible spiny proboscides.
4. Vitellaria in cortical parenchyma placed in continuous layer.
5. Testes extend behind the ovary posteriorly.
6. Gonopores lateral; uterus opens ventrally.
Examples:
Haplobothrium, Tetrarhynchus.
Order 4. Pseudophyllidea:
1. Parasitic in the intestine of teleost fishes and terrestrial vertebrates.
2. Body segmented into strobila or un-segmented.
3. Scolex with two to six shallow bothria (suckers) rarely without adhesive organs.
4. Ovary bilobed, testes numerous, follicular and scattered in the mesenchyma of proglottids.
5. Vitellaria follicular, numerous.
6. Gonopores midventral.
Examples:
Bothriocephalus, Dibothrio- cephalus.
Order 5. Taenioidea or Cyclophyllidea:
1. Parasitic in the intestine of reptiles, birds and mammals.
2. Large-sized tapeworms.
3. Scolex bears four large in-cupped suckers (acetabula) often with an apical rostellum armed with hooks.
4. Ovary two to many lobed; uterine opening absent.
5. Gonopores on one or both margins.
6. Excretory system consists of four longitudinal vessels.
7. Vitellaria (yolk gland) single and compact.
Examples:
Taenia, Echinococcus, Hymenolepis, Moniezia.
