PHYLUM PLATYHELMINTHES

Phylum Platyhelminthes (Gr., platys = flat ; helmins = worms) refers to a group of invertebrate animals commonly known as flatworms. They are characterized by their soft, unsegmented, dorsoventrally flattened bodies. Members of this phylum are bilaterally symmetrical, triploblastic (having three embryonic germ layers), and acoelomate, meaning they lack a true body cavity. Platyhelminthes exhibit cephalization (a distinct head region) and possess a simple organ system level of organization.

General Characteristics of Phylum Platyhelminthes

• Tissue-organ level of organization.
• Platyhelminthes are bilaterally symmetrical and dorso-ventrally flattened worms.
• Triploblastic body. Third germ layer mesoderm instead of mesoglea.
• Anterior end of the body is differentiated into head.
• Presence of great variety of adhesive secretions, organs of attachment and adhesion (eg., hooks and suckers).
• Exo- and endoskeleton are completely absent.
• Acoelomate, i.e., true coelom is absent.
• More complex and efficient gastrovascular cavity.
  In some flatworms digestive system is totally absent but in other it consists of mouth, pharynx and blind intestine.
• Respiratory and circulatory systems are absent.
• Excretory system consists of single or paired protonephridia with flame cells or bulbs.
• Nervous system is primitive with large, anteriorly situated ganglia and nerve cords running along the body.
• Better developed sensory organs. Chem- and tango- receptors commonly occur in form of ciliated pits and grooves.
• Sexes are united. i.e., hermaphrodite, with very few exceptions.
  Internally situated gonads with permanent gonoducts and copulatory structures.
• Life cycle complicated, involves one or more hosts.
• Parthenogenesis and polyembryony are of common occurrence.
• Behaviour organization through learning.

Classification of Phylum Platyhelminthes

Note: Classification is adopted from Hyman, L.H. (1951) up to suborders with certain modifications.

CLASS 1. TURBELLARIA

1. Turbellaria are mostly free-living-flatworms but some are ectocommensals or endocommensals or parasitic.
2. Body unsegmented.
3. Body covered with a cellular or syncytial epidermis which is usually ciliated. Epidermis is provided with rhabdoids.
4. Turbellaria are abundantly provided with adhesive organs and which are of two types (i) glandula epidermal adhesive organs and (ii) glandula muscular adhesive organs.
5. Digestive system consists of mouth, pharynx and intestine except in Acoela in which intestine is absent.
6. Excretory system consists of protonephridia provided with terminal flame bulbs.
7. Sense organs consist of tangoreceptors and chemoreceptors.
8. Hermaphroditic or sexes separate with very few exceptions.
9. Reproduction sexual, asexual and by regeneration.
10. Life-cycle simple.

Order 1. Acoela

1. Small turbellarian exclusively marine.
2. Digestive system consists of mouth and simple pharynx.
3. Intestine is absent.
4. Excretory system totally absent.
5. Reproductive system without oviducts and yolk glands.
6. Example: Convoluta.

Order 2. Rhabdocoela

1. Small freshwater, marine and terrestrial forms.
2. Digestive tract is complete, intestine sac-like without diverticula.
3. Excretory system consists of protonephridia.
4. Reproductive system comprises few compact gonads, oviducts and a cuticularized structure instead of penis papilla.
5. Nervous system with two main longitudinal trunks.
6. Free-living, commensal or parasitic forms.

Suborder 1. Notandropora

1. Exclusively freshwater forms.
2. Simple pharynx.
3. Excretory system consists of single median protonephridia.
4. Single compact testis mass, unarmed penis.
5. Yolk gland is absent.
6. Asexual fission with formation of chains of zooids.
7. Examples: 1. Catenula, 2. Rhycoscolex.

Suborder 2. Opisthandropora

1. Freshwater or marine forms.
2. Pharynx simple.
3. Excretory system consists of paired nephridia.
4. Compact testes, penis armed with a stylet.
5. Yolk glands absent.
6. Asexual reproduction with chains of zooids.
7. Examples: 1. Macrostomum, 2. Microstomum.

Suborder 3. Lecithopora

1. Freshwater, marine or terrestrial Turbellaria.
2. Pharynx bulbose.
3. Excretory system consists of paired protonephridia.
4. Separate ovaries and yolk glands.
5. Reproduction exclusively sexual.
6. Examples: 1. Anoplodium, 2. 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. Simple gonopore.
6. Examples: 1. Temnocephala, 2. Monodiscus.

Order 3. Alloecoela

1. Mostly marine, few freshwater and brackish-water forms. Some species are ectoparasitic or ectocommensals in habit.
2. Pharynx simple, bulbose or plicate.
3. Intestine with short diverticula.
4. Excretory system consists of paired protonephridia having two or three main branches and nephridiopores.
5. Reproductive system consists of numerous small testes and a pair of ovaries.
6. Penis papilla usually present.
7. Nervous system with three or four pairs of longitudinal trunks provided with transverse connectives.

Suborder 1. Archophora

1. Marine forms.
2. Pharynx plicate.
3. Female reproductive system primitive.
4. Example: Proporoplana.

Suborder 2. Lecithoepitheliata

1. Marine, freshwater or terrestrial forms.
2. Pharynx simple or bulbose.
3. Penis with a cuticular stylet.
4. Female duct simple or none.
5. Yolk gland absent. Nutritive cells surround the ova.
6. Example: Prorhynchus.

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.
6. 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.
6. Examples: 1. Otoplana, 2. Bothrioplana.

Order 4. Tricladida

1. Marine, freshwater or terrestrial forms.
2. Pharynx plicate usually directed backwards.
3. Intestine with three highly diverticulated branches.
4. Male reproductive organs consist of two to numerous testes.
5. Female reproductive organs comprise a pair of ovaries with yolk glands.
6. Male apparatus provided with a penis papilla and female apparatus with a bursa.
7. Gonopore single.

Suborder 1. Maricola

1. Exclusively marine forms.
2. A pair of eyes and auricular grooves.
3. Typical penis papilla sometimes armed with stylet.
4. Rounded copulatory bursa is generally present.
5. Only sexual reproduction.
6. Examples: 1. Bdelloura, 2. Ectoplana, 3. Gunda segmentina.

Suborder 2. Paludicola

1. Mostly freshwater, rarely brackish water forms.
2. Eyes two to many or absolutely absent.
3. Bursa usually present anterior to penis.
4. Reproduction mostly asexual.
5. Example: Planaria or Dugesia.

Suborder 3. Terricola

1. Terrestrial tropical and subtropical forms.
2. Body mostly elongated with two to many eyes.
3. Bursa mostly absent.
4. Male and female antra usually separate.
5. Reproduction may be asexual.
6. Examples: 1. Bipalium, 2. Geoplana.

Order 5. Polycladida

1. Exclusively marine.
2. Body usually broad and flattened, some are elongated.
3. Pharynx plicate opens into main intestine.
4. Intestine with numerous branches.
5. Eyes numerous.
6. Nervous system consists of numerous radiating nerve cords.
7. Testes and ovaries are numerous and scattered.
8. Yolk glands absent.
9. Gonopore one or two.

Suborder 1. Acotylea

1. Pharynx usually vertical curtain-like.
2. Sucker absent behind the female pore.
3. Tentacles nuchal type.
4. Eyes never occur as a pair of clusters on the anterior margin.
5. 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.
4. Examples: 1. Thysanozoon. 2. Yungia.

CLASS 2. TREMATODA

1. Ecto- or endoparasitic forms.
2. Body shape usually leaf-like.
3. Body wall without epidermis and cilia.
4. Body is undivided and covered with a cuticle.
5. Well developed suckers are usually present.
6. Digestive tract complete consisting of mouth, pharynx and intestine.
7. Excretory organs are protonephridia.
8. Mostly hermaphroditic except few families of Digenea.
9. Ovary single. Testes two to many.
10. Life-cycle simple or complicated.

Order 1. Monogenea

1. Ecto- or endoparaitic forms of vertebrates.
2. Oral sucker weak or absent.
3. Anterior end usually with a pair of adhesive structures.
4. Posterior end provided with an adhesive disc usually with hooks.
5. Excretory pores paired.
6. Uterus short containing few eggs.
7. Life-cycle simple, no alternation of hosts.
8. Examples: 1. Gyrodactylus, 2. Polystoma, 3. Diplozoon.

Order 2. Aspidobothria

1. Endoparasitic forms.
2. Oral sucker absent.
3. Anterior end without paired adhesive structures.
4. Large ventral sucker subdivided into compartments without hooks.
5. Nephridiopore is single.
6. Life-cycle simple, no alternation of hosts.
7. Example: Aspidogaster.

Order 3. Digenea

1. Endoparasitic forms.
2. Mostly with two suckers without hooks.
3. Single excretory pore.
4. Uterus usually long with many eggs.
5. Life-cycle complicated with many larval stages and one or more intermediate hosts.
6. Examples: 1. Fasciola, 2. Schistosoma, 3. Bucephalus, 4. Clonorchis (Opisthorchis); 5. Gastrothylax, 6. Paramphistomum.

CLASS 3. CESTODA

1. Endoparasitic in the intestine of vertebrates.
2. Cestoda are commonly known as tapeworms.
3. Body is without epidermis and cilia but covered with a cuticle.
4. Body is usually divided into few to many segments (proglottids) rarely undivided.
5. Anterior end is provided with adhesive structures (hooks, suckers) except in Cestodaria.
6. Mouth and digestive tract are 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 (proglottid) contains one or two sets of complete hermaphroditic reproductive system.
10. Life-cycle complicated with hooked embryo and usually involving two or more hosts.

Subclass 1. CESTODARIA

1. Endoparasitic in coelom or intestine of vertebrates.
2. Body is undivided.
3. Scolex absent.
4. Only single set of reproductive organs.
5. Larvae with 10 hooks.

Order 1. Amphilinidea

1. Endoparasitic in the coelom of fishes.
2. Body is flattened, oval or elongated.
3. Scolex absent.
4. Anterior end bears protrusible proboscis and frontal glands.
5. Male and vagina pore posterior.
6. Uterus is very much coiled opening near anterior end.
7. Example: Amphilina.

Order 2. Gyrocotylidea

1. Endoparasitic in the intestine of fishes.
2. Body is elongated and flattened.
3. Anterior end bears reversible proboscis.
4. Posterior end with adhesive structures.
5. Uterine, male and vaginal pores are together situated in the anterior half of the body.
6. Uterus short, runs directly to pore.
7. Example: Gyrocotyle.

Subclass II. EUCESTODA

1. Body is usually very elongated and ribbon-like.
2. Body divided into few to many segments rarely undivided.
3. Anterior end bears an expanded scolex bearing adhesive structure.
4. Each segment (proglottid) with more than one set of reproductive organs.
5. Larva with six hooks.

Order 1. Tetraphyllidea

1. Endoparasitic 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. Cirrus armed with hairs, spines or hooks.
5. Examples: 1. Phyllobothrium, 2. Myzophyllobothrium.

Order 2. Diphyllidea

1. Parasitic in the intestine of Elasmobranch fishes.
2. Tapeworms of not more than 20 segments.
3. Scolex with two bothria and a spiny head stalk.
4. Example: Echinobothrium

Order 3. Trypanorhyncha

1. Parasitic in the spiral valve of digestive tract of Elasmobranch fishes.
2. Body always segmented and of moderate size.
3. Scolex provided with four bothria and four protrusible spiny proboscides.
4. Examples: 1. Haplobothrium, 2. Tetrarhynchus.

Order 4. Pseudophyllidea

1. Endoparasitic in the intestines of vertebrates.
2. Body segmented or unsegmented.
3. Scolex with two to six shallow bothria (suckers) rarely without adhesive organs.
4. Yolk glands numerous.
5. Examples: 1. Bothriocephalus, 2. Dibothriocephalus or Diphyllobothrium.

Order 5. Taenioidea or Cyclophyllidea

1. Endoparasitic in the intestine of birds and mammals.
2. Large tapeworms always with segmented body.
3. Scolex bears four suckers (acetabula) often with an apical rostellum armed with hooks.
4. Excretory system consists of four longitudinal vessels.
5. Yolk gland single and compact.
6. Examples: 1. Taenia solium, 2. Echinococcus, 3. Hymenolepis nana, 4. Dipylidium caninum, 5. Raillietiana, 6. Cotugnia.