In this article we will discuss about Phylum Platyhelminthes:- 1. Taxonomic Retrospect of Phylum Platyhelminthes 2. Definition of Phylum Platyhelminthes 3. Characteristic Features 4. Scheme of Classification.
Taxonomic Retrospect of Phylum Platyhelminthes:
Linnaeus in his Systema Naturae (1735) created a phylum Vermes in which he included all invertebrates except the insects but in the 13th ed. of Systema Naturae (1788) all flat worms were placed under the order Intestina.
Zeder (1800) first enlisted the parasitic worms.
Lamarck (1816) separated between the flatworms and roundworms.
Cuvier (1817) included the flatworms within Zoophyta or Radiata.
Vogt (1851) called flatworms as Platyelmia.
Gegenbauer (1859) first coined the name platyhelminthes and divided the phylum platyhelminthes into 4 classes:
(i) Turbellaria,
(ii) Trematoda.
(iii) Cestoda and
(iv) Nemertea but Minot (1876) excluded the nemerteans from the Platyhelminthes.
Definition of Phylum Platyhelminthes:
Triploblastic, acoelomate, un-segmented and bilaterally symmetrical metazoans, without anus, circulatory system, but with a mouth, protonephridial system, parenchyma tissues within the space between the body wall and the gut, and spiral cleavage development.
Characteristic Features of Phylum Platyhelminthes:
1. Phylum Platyhelminthes are triploblastic, acoelomate (without a body cavity) and bilaterally symmetrical animals.
2. Body is soft, un-segmented and dorsoventrally flattened.
3. Metameric segmentation and skeletal structures, in any form, are absent. Pseudometamerism is seen in some members (e.g., Eucestoda).
4. Anterior end of the body is differentiated into a head.
5. The body is covered by epidermis which is soft, syncytial and ciliated (e.g., Turbellaria) or in rest classes (e.g., Trematoda, Monogenea and Cestoidea) there are a syncytial, non-ciliated body covering, called tegument.
6. The parenchyma tissue, a mesenchymal layer which occupies the entire space between the outer body wall and the endoderm of the gut, develops from both ectodermal and entomesodermal layer of the embryo.
7. The mouth of Phylum Platyhelminthes is the single opening of the digestive tract and the anus is absent when the digestive canal is present. The digestive system is totally absent in Cestoidea and Acoela, and digestive canal is branched in turbellarians. So the digestive system is incomplete when present.
8. Locomotion of Phylum Platyhelminthes is effected by the use of cilia rather than muscles.
9. There are no respiratory and circulatory systems. Haemoglobin is present in a very few species (e.g., Derostoma, Syndesmis, Telorchis, etc.).
10. The nervous system is most primitive type and is formed by longitudinal nerve cords with a pair of cerebral ganglion or brain.
11. The excretory system consists of protonephridia with flame cells or bulbs.
12. Reproductive system is generally hermaphroditic but the digenean flukes are gonochoristic (separate sexes).
13. Fertilization internal.
14. Cleavage spiral, and determinate in some species.
15. Development may be direct or indirect.
16. Flatworms are either mostly free-living (Turbellarians) or are internal or external parasites (Trematodes, cestodes and monogeneans).
17. Parthenogenesis and polyembryony usually occur in trematodes and tapeworms.
Scheme of Classification of Phylum Platyhelminthes:
The scheme of classification is based on partly, outlined by Ruppert and Barnes (1994), taken from the book “Invertebrate Zoology”.
Phylum Platyhelminthes
Class Turbellaria
Class Trematoda
Subclass Digenea
Subclass Aspidogastrea (Aspidobothria)
Class Monogenea
Class Cestoidea (= Cestoda)
Subclass Cestodaria
Subclass Eucestoda
Classification with Characters:
Class 1. Turbellaria (Gk. turbella = a little string); Approx. 3000 species.
Features:
1. Mostly free-living, primarily aquatic and the great majority are marine and mostly benthic; a few are terrestrial, some are commensals for parasites.
2. Generally small, flattened, bilaterally symmetrical with a low level of cephalization.
3. Un-segmented body.
4. Cilia scattered on epidermis.
5. Locomotion by cilia with muscular undulations.
6. Body surface of many species bears small, rod-like, hyaline rhabdites and related rhabdoids—unique in turbellarians and the function is uncertain.
7. Presence of epidermal gland cells (Duo-gland, frontal gland, rhabdite gland) help for adhesion, mucus secretion and other secretory functions.
8. Pigmented and some are brilliantly coloured.
9. Simple sac-like intestine with a simple or bulbous pharynx in small-size species, and in large-size species the intestine is branched with a tubular pharynx.
10. Body surface acting as a permanent respiratory surface.
11. Suckers absent.
12. Nervous system is a diffuse nerve-net with a cerebral ganglion, a primitive type found in some forms and a distinct brain with 1-3 pairs of longitudinal nerve cords found in advanced forms.
13. Pigment cup ocelli and statocysts are the sensory organs.
14. Excretory system is protonephridia which includes flame cells in most cases.
15. Mostly hermaphrodite.
16. Asexual reproduction in many species (e.g., freshwater and terrestrial turbellarians).
17. Spiral cleavage in many forms (e.g., in some acoels and polyclads),
18. Generally development direct.
19. A free-swimming larval stage Muller’s larva is found in some forms (e.g., a few entolecithal polyclads).
It includes the following major orders:
The orders of the class Turbellaria are divided into two groups on the basis of their organizations.
A. Archoophoran turbellaria:
1. Primitive type of organization.
2. Vitellaria (or called yolk gland) absent.
3. Eggs entolecithal.
Order 1. Nemertodermatida:
1. Free-living marine, small species resemble the acoels in many respects.
2. Single flagellated sperm possesses 9 + 2 microtubular arrangements.
3. Presence of a permanent epithelial gut cavity.
4. Statocysts usually with two staloliths rather than one.
Example. Nemertoderma.
Order 2. Acoela:
1. Small marine turbellarians.
2. Mouth and pharynx present but no digestive cavity.
3. Protonephridia and gonads absent.
4. Statocyst with a single statolith.
5. A few species are commensals with other invertebrates (e.g., Ectocotyla paguris)
Examples. Amphiscolopus, Afronta, Convoluta, Archayhanostoma, Paratomella.
Order 3. Catenulida:
1. Mostly small freshwater species, although a few are marine.
2. Simple pharynx.
3. Ciliated, sac-like intestine.
4. Unpaired median protonephridium.
5. Unpaired gonads.
6. Statocyst sometimes present.
Examples. Stenostomum, Catenula.
Order 4. Macrostomida:
1. Most species are interstitial and live in sand grains of either marine or freshwater sediments.
2. Pharynx simple and sac-like intestine.
3. Paired protonephridia.
4. Numerous adhesive glands are present in most species.
5. Statocysts are absent.
Examples. Microstomum, Macrostomum.
Order 5. Polycladida:
1. Mostly benthic marine of moderate size; some pelagic and some symbiotic.
2. Eyes numerous.
3. Ruffled plicate pharynx in most cases.
4. Highly branched gut.
5. Many are brightly coloured.
6. Greatly flattened and oval in shape.
Examples. Stylochus, Adenoplana, Notoplana, Hoplolana, Planocera, Pseudoceros.
B. Neoophoran turbellaria:
1. Advanced level of organization.
2. Vitellaria (yolk gland) present.
3. Eggs ectolecithal.
Order 1. Lecithoepitheliata:
1. Marine, freshwater and terrestrial species.
2. Mouth and simple or bulbous pharynx are at the anterior end of the body.
3. Simple intestine.
4. Penis with cuticular stylet.
5. Eggs are surrounded by yolk cells.
Example. Prorhynchus, Hofstenia.
Order 2. Prolecithophora:
1. Marine and freshwater species.
2. Plicate (folded) or bulbous pharynx.
3. Simple intestine.
4. Penis unarmed.
5. Ovary produces eggs and follicle-like yolk cells.
6. Some species are ectoparasites on certain decapod crustaceans.
Example. Plagiostomum.
Order 3. Proseriata:
1. Mostly marine.
2. Pharynx plicate and tubular.
3. Gut simple.
4. Statocysts present.
5. Ovaries and yolk glands separate.
Examples. Otoplana, Nemertoplana, Bothrioplana.
Order 4. Rhabdocoela:
1. Small marine or freshwater turbellarians.
2. Bulbous pharynx.
3. Digestive tract straight.
4. Protonephridia either 1 or 2.
5. Colourless.
6. Commensal or parasitic; some are with tentacles.
Examples. Graffilla, Mesostoma, Syndesmis, Gyratrix, Temnocephala.
Order 5. Tricladida:
1. Marine, freshwater and terrestrial forms.
2. Relatively large-size species.
3. Mouth mid-dorsal and with proboscis.
4. Plicate pharynx.
5. Three-branched digestive tube.
6. Eye spots present.
7. Two ovaries and two or numerous testes.
8. Mostly free-living.
The freshwater species of this order are known as planarians.
Examples:
Bdelloura (commensal on the book gills of horse-shoe crabs), Polycelis, Dugesia (previously called Planaria, common in ponds, lakes and streams), Euplanaria, Bipalium and Geoplana (terrestrial triclads), Procotyla, Planaria (Northern Hemisphere), Dugesia can differ from Planaria by the presence of eyes with pigment cups.
Class 2. Trematoda (Gk. trematodes = perforated):
Features:
1. All are parasites.
2. Unsegmented, dorso-ventrally flattend body, hence trematodes are called “flukes”.
3. A syncytial, non-ciliated body covering or tegument present.
4. Rhabdites absent in epidermis.
5. One or more well developed suckers are present. Oral sucker for feeding and ventral sucker (acetabulum) for attachment.
6. Digestive tract complete with two branches.
7. Mostly hermaphrodites.
8. No asexual reproduction.
9. In most Cases the testes are two or many, but always single ovary.
10. Egg develops as a ciliated miracidium larva which infects the intermediate host (molluscan).
11. Most are endoparasites and have two or three hosts in the life cycle.
It includes two subclasses—(i) Digenea (ii) Aspidobothria (= Aspidogastrea)
Subclass 1. Digenea (Gk. di = two + genos = a race, kind; Approx. 11200 species).
Features:
1. Flattened, oval to elongated body.
2. Mostly with two suckers; mouth within oral sucker and a ventral sucker or acetabulum which is usually larger.
3. Hooks absent.
4. Gut present. Two blind intestinal caeca.
5. Single excretory pore situated posteriorly.
6. Usually two testes.
7. Uterus long and tubular.
8. Life cycle is completed with at least two hosts. The adults occur in the definitive host (normally a vertebrate) and larval stages within intermediate hosts (gastropod snails), If there is a second intermediate host, they are usually an arthropod.
9. Chiefly endoparasite.
Examples. Fasciola, Schistosoma (Blood fluke), Clonorchis.
Subclass 2. Aspidogastrea (= Aspidobothria); Approx. 35 species.
Features:
1. Oral sucker or anterior adhesive structures absent.
2. Whole ventral surface of the body has a big septate sucker or rows of suckers.
3. Single posterior protonephridium present.
4. Digestive tract with a single intestinal caecum.
5. Typically single testis present.
6. Mostly endoparasites in the gut of fish, reptiles or in the renal or pericardial cavities of gastropodes and bivalve molluscs (fresh-water mussels).
7. Simple life cycle with a single host in most cases.
Examples:
Aspidogaster, Stichocotyle, Cotylapsis.
Class 3. Monogenea (Gk. monos = single + genos = a race)
Features:
1. Ectoparasites of aquatic vertebrates (on fishes and also on amphibians and reptiles).
2. Dorsoventrally flattened body.
3. Sucker is weak or absent.
4. Posterior end bears a large specialized adhesive disc, called haptor (= opisthaptor) which bears hooks and suckers, used for attachment. In addition there is an anterior adhesive organ, called prohaptor, with adhesive glands and suckers.
5. Gut present but the mouth lacks a sucker.
6. Inconspicuous protonephridia include paired excretory pores situated anterodorsally.
7. Single host in the life cycle.
8. There is no intermediate host and the one egg gives rise to only one adult worm, hence the name “Monogenea”.
9. Ciliated Oncomiracidium larva in the life cycle which bears 3 bands of cilia and 1 to 2 pairs of eyes.
There is no subclass or order under Monogenea.
Examples. Gyrodactylus, Polystoma, Sphyranura, Dactylogyrus, Diplozoon.
Class 4. Cestoidea (= Cestoda) (Gk. kestos – a girdle, L. cestus = ribbon, eidos = form; Approx. 3500 sp.
Features:
1. Gut parasites of the vertebrates.
2. Generally called tape-worms.
3. Body covered by a non-ciliated syncytial tegument.
4. Body divided into 3 regions:
(i) Head or scolex usually with hooks and suckers for adhesion in the host’s gut,
(ii) The proliferative neck and
(iii) Strobila which consists of a large number of segments, called proglottids.
5. Mouth, digestive tract and sense organs absent.
6. Organs of attachment in the form of hooks and suckers are present.
7. Each segment excepting head and neck is provided with one or two sets of complex sex organs.
8. Hermaphrodite.
9. Hooked embryos, called Oncosphere larva.
10. Complicated life cycle with one or more intermediate hosts.
It includes two subclasses:
Subclass 1. Cestodaria:
Features:
1. Body small and undivided.
2. No scolex; proboscis or sucker at the anterior part of the body.
3. Proglottids absent (unstrobilated cestodes).
4. One set of reproductive structure present.
5. Larva with 10 hooks.
6. Gut parasite of cartilaginous fishes.
It includes two orders:
Order 1. Amphilinidea:
1. Proboscis protrusible.
2. Frontal glands are placed at anterior end.
3. Genital apertures of both sexes posterior in position.
4. Uterus extensive in length and traverses the body.
Examples. Amphilina (in the fish, sturgeon).
Order 2. Gyrocotylidea:
1. Anterior end with eversible proboscis.
2. Posterior end with adhesive disc.
3. Uterus short, runs directly to the pores.
4. Endoparasites in chimaeroid fishes.
Example. Gyrocotyle in chimaeroid fish.
Subclass 2. Eucestoda:
Features:
1. Body long, ribbon-like and segmented (strobilated).
2. Scolex bears adhesive organs such as suckers and hooks.
3. Scolex include rows of hooklets forming a rostellum (a conical elevation of the scolex), muscular suckers, elongated flaps of tissue, called bothria.
4. Each proglottid contains more than one set of reproductive organs.
5. Larva bears six hooks.
6. Life cycle completes by one, two or sometimes more intermediate hosts.
It includes the following orders:
Order 1. Caryophyllidea:
1. Body unsegmented (no proglottids).
2. Scolex ill-defined and usually no suckers or bothria.
3. Presence of only one set of male and female organs.
4. Parasites in freshwater fish and oligochaetes (adults) and freshwater oligochaetes (intermediate hosts).
Examples. Archegetes, Caryophyllaeides.
Order 2. Spathebothridea:
1. Body unsegmented (no proglottids).
2. Lacking of any attachment organs (suckers), including a scolex.
3. Reproductive organs are arranged in a linear series.
4. Parasites in marine teleosts.
Examples. Spathebothrium, Cyathocephalus.
Order 3. Lecanicephaloidea:
1. Scolex differentiated into two parts, upper with disc or branches and lower with 4 suckers.
2. Intestinal parasites of elasmobranch fishes.
Examples. Polypocephalus (parasite in elasmobranch fishes), Lecanicephalum.
Order 4. Tetraphyllidea:
1. Scolex bears 4 longitudinal slits, called bothridia, often with hooks.
2. Common genital atrium marginal.
Examples. Phyllobothrium, Myzophyllobothrium (live as parasite of elasmobranchs) and other members also found in amphibians and reptiles.
Order 5. Proteocephaloidea:
1. Scolex mobile with 4 lateral and one apical suckers.
2. Ovary bilobed. Uterus with many lateral branches.
3. Common genital atrium marginal.
Example. Proteocephalus (lives as parasites in the intestine of fish, amphibia and reptile).
Order 6. Disculiceptidea:
1. Scolex provided with cushion-like pad without suckers and hooks.
2. Proglottids square-shaped.
3. Common genital atrium ventral.
4. Parasites in sharks.
Example. Disculiceps.
Order 7. Diphyllidea:
1. Scolex large and lobed with two pairs of suckers.
2. Neck with 8 rows of hooks.
3. Vitellaria scattered.
Example. Echinobothrium (parasitize elasmobranch fish).
Order 8. Trypanorhyncha:
1. Scolex with 4 lobes and bothridia.
2. Spiny protrusible proboscis.
3. Vitellaria in a continuous layer in cortical parenchyma.
Examples. Haplobothrium, Grillotia, Lacistorhyncus, Tetrarhynchus (parasitize the elasmobranch fish).
Order 9. Pseudophyllidea:
1. Scolex indistinct.
2. 2-6 shallow groove-like suckers or bothria.
3. Hooks usually absent.
4. Vitellaria numerous and follicular.
5. Genital apertures midventral.
Examples. Diphyllobothrium, Spirometra, Polygonoporus, Dibothriocephalus (mostly parasitize fishes, some lives as parasite in amphibians and birds).
Order 10. Nippotaeniidea:
1. Small.
2. No scolex.
3. One apical sucker present.
4. Parasites in freshwater fishes.
Example. Nippotaenia.
Order 11. Taenoidea or Cyclophyllidea:
1. Scolex with 4 cup-shaped suckers.
2. Usually with one or two rows of hooks at tip.
3. Sex aperture lateral in position.
4. Yolk gland single and compact.
5. Large-sized tape-worms.
Examples. Taenia, Echinococcus, Hymenolepis, Vampirolepis, Dipylidium (Larva found in Crustacea, Insecta and Vertebrata, and adults found in vertebrates).
Order 12. Aporidea:
1. Scolex with 4 suckers and armed rostellum.
2. No yolk gland.
3. Parasites in birds.
Examples. Gastrotaenia (found in swan), Nematoporataenia.
