Characters and classification of platyhelminthes.

Character:

• Free-living commensal or parasitic forms.
• Tissue-organ grade of organization, I.e, body cells aggregates into definite tissue and tissues make up endoderm.
• Triploblastic,i.e., body derived from three embryonic germ layers; ectoderm,endoderm,and mesoderm.
• Bilaterally symmetrical with the definite polarity of anterior (head) and posterior (tail ends).
• Dorso-ventrally flattened. Usually with a well-defined ventral surface bearing mouth and gonopore.
• Body Unsegmented (except) in class Cestoda).
• Adhesive structures like hooks, spines, and suckers, and adhesive secretions common in parasitic forms.
• Epidermis cellular or syncytial, frequently ciliated.Absent in some.
• Muscular system of mesodermal origin.Longitudinal, circular and oblique muscle layers beneath the epidermis.
• Digestive system branched and incomplete without an anus.Altogether absent in Acoela and Cestoda.
• Skeletal, respiratory and circulatory systems are wanting.
• The excretory system includes lateral canals and protonephridia (flame shell). Absent in some primitive forms.
• Nervous system primitive, ladder-like.Comprises a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves.
• Sense organ simple.Eye-spots or photoreceptors in free forms.
• Mostly monoecious (hermaphrodite) with the complex reproductive system.Well-developed gonads, gonaducts and accessory organs.Eggs mostly devoid of yolk.Yolk produced separately in the yolk or vitelline glands.
• Fertilization internal may be cross or self.
• Development direct or indirect. Usually indirect in endoparasites with a complicated life cycle involving many larvae and hosts.

Classification:

Class1.Turbellaria

(L.,turbinella, a stirring)

1. Usually none- parasitic, free-living worms are called planarians.
2. Terrestrial-marine of freshwater.
3. Body unsegmented flattened and covered with ciliated cellular or syncytial epidermis, containing mucus-secreting cells and rod-shaped bodies called rhabdites.
4. Mouth ventral. Intestine preceding by muscular pharynx.
5. Sucker absent. Tango, chemo and photo-receptors common in free-living forms.
6. Mostly hermaphroditic. Some reproduce asexually. Development usually direct.

Order 1. Acoela

1. Minute, exclusively marine, less than 2 mm.
2. Central mouth; no muscular pharynx and without intestine.
3. Flame shell, definite gonads, gonaducts and yolk glands wanting.
4. Mostly free- living, found under stones, algae or on bottom mud. Some dwell in the intestine of sea urchins and sea cucumbers.

Examples:Convoluta, Amphiscolops, Ectocotyl, Sea-cucumbers.

Order 2. Rhabdocoela

1. Small, Usually less than 3 mm.
2. Simple pharynx and sac-like intestine
3. Protonephridia excretory system.
4. One or two gonads. Yolk glands present or absent.
5. Marine, freshwater or terrestrial. Free-living commensal or parasitic.

Examples: Stenostomum, Microstomum, Action dactyletta, Catenula, Macrostomum, Mesostoma.

Order 3. Alloecoela

1. Moderate-sized, between 1 and 10mm.
2. Pharynx simple, bulbous or plicate.Intestine straight or branched.
3. Protonephridia paired, usually branched.
4. Testes numerous. Penis papilla mostly present.
5. Mostly marine, common in littoral sand and mud. Some freshwater.

Examples: Oxyrhynchus, plagiostomous, Geocentrophora.

Order4. Tricladida

1. Large 2 to 6o cm in length.
2. Mouth mid-ventral, pharynx, plicate and intestine with three branches, each with much diverticular.
3. Protonephridia as lateral network with many nephridiopore.
4. Tested numerous, ovaries two. Yolk glands present.
5. Marine. freshwater or terrestrial.

Examples: Dugesia, Gunda, Bdelloura, Euplanaria, Bipalium.

Order 5. polycladida

1. Modern-sized,2 to 20 mm.
2. Pharynx plicate. Intestine highly branched.
3. Gonads many, scattered.Yolk glands separate.
4. Male and female gonophores separated.
5. Marine, many bottom dwellers of the littoral zone.

Examples: Leptoplana, notoplana, cestoplana, planocera.

Class 2. Trematoda

( Gr.,tremta,hole+epidos, form)

1. Ecto- or endoparasitic flatworms, called flukes.
2. Body segmented, dorsoventrally flattened, leaf-like. Tegument thick but without cilia and rhabdites.
3. Sucker and sometimes hook present.
4. An alimentary canal with the anterior mouth, simple pharynx, and two main branches.
5. Three pairs of longitudinal nerve cords.
6. Mostly monoecious. Development direct (in ectoparasites) or indirect (in endoparasites) with alternating of hosts.

Order 1.Monogenea

1. Mostly ectoparasites in cold-blooded aquatic vertebrates.
2. Posterior adhesive organ or opisthaptor with suckers with hooks or spines.
3. Excretory pores two situated anteriorly on the dorsal side.
4. Vagina one or two. Uterus small with a few shelled eggs.
5. Only a single host in the life cycle.

Example: Gyrodactylus,Dactyogyrus, Polystoma, Diplozoon.

Order 2. Digenea

1. Mostly endoparasite in vertebrates and invertebrates.
2. Two suckers, oral and acetabulum, both devoid of hooks.
3. Single posterior excretory pore.
4. No vagina. Uterus long with numerous shelled eggs.
5. Lifecycle complex with numerous larval stages in two to three intermediate hosts.

Examples: Bucephalus, Fasciola, Param- Philtrum, Paragonimus.

Order 3. Aspidocotylea

1. Oral sucker absent.
2. Large ventral sucker subdivided into several suckers without hooks.
3. Only one testis in the male system.
4. Endoparasites in the gut of fishes and reptiles.

Examples: Aspidogaster, Cotylapsis,

Class 3. Cestoda

1. Endoparasitic flatworms called tapeworm.
2. Body segmented, elongated, flat, ribbon-like.
3. Scolex (head) with sucker, or hooks,or both.
4. No alimentary Canal.No sense organs.
5. Lifecycle complicated involving one or more intermediate host.

Subclass A. Cestodaria

1. Body unsegmented, leaf-like, without scolex and (mono zoic).
2. Only one set of the monoecious reproductive system.
3. Larva lycopene, with 10 hooks.

Order 1. Amphilinidea

1. No suckers.Pharynx protrusible.
2. Male genital pore and vegina situated posteriorly. Uterus coiled.
3. Endoparasitic in celom of primitive fishes.

Example:

Order 2.Gyrocotylidea

1. An anterior sucker, a posterior roselle shaped adhesive organ present.
2. Eversible proboscis at the anterior end.
3. Endoparasites in chimaeroid fishes.

Example:Gyrocotyle.

SubclassB. Eucestoda

1. Body long ribbon-like . Divided into scolex, neck and strobila with many proglottids (polyzoic).
2. Mostly with six hooks .
3. Mostly with several sets of monoecious.

Order 1.Proteocephalidea

1. Scolex with 4 cup-shaped suckers.
2. Ovary bilobed.
3. Uterus branched.
4. Vitellaria scattered.
5. Parasitic in freshwater fishes, amphibians,fishes, and reptiles.

Example:Proteocephalus.

Order.2 Tetraphyllidea

1. Scolex with 4 leaflike bothria.
2. Testes anterior to ovaries. Vitelline glands scattered.
3. Parasitic in the intestine of elasmobranch fishes.

Examples; Phyllobothrium

Order 3. Disculicepitidea

1. Scolex with large cushion shaped pad at the anterior end.
2. Femalr gonopore, anterior to male gonopore.
3. Endoparasitic of Selachii.

Examples:Disculiceps.

Order 4. Lecanicephaloidea

1. Scolex divided by a transverse groove.
2. Up:per disc-like lower with 4 suckers.
3. Vitellaria as two lateral bands.
4. Intestinal parasites in elasmobranch fishes.

Examples Lecanicephalum.

Order 5.Pseudophyllidea

1. Scolex with 2 to 6 bothria.
2. Tested numerous.Ovary bilobed.
3. Parasitic in freshwater fishes.

Example: Dibothriocephalus.

Order 6. Trypanorhyncha

1. Scolex with 2 to 4 bothria and 4 spiny tentacles.
2. Vitellaria in continuous layer in cortical parenchyma.
3. Parasitic in elasmobranch fishes.

Examples: Tetrahynchus.

Order 7. Cyclophyllidea

1. Scolex with 4 large deep suckers and hooks.O
2. Ovary lobed . uterus blind.
3. Parasitic in amphibian, reptiles, birds, and mammals.

Example: Taenia, echinococcus.

Order 8. Apoidea

1. Scolex with 8 suckers.
2. No external segmentation.
3. Ootype absent.Vitellaria absent or present.
4. Parasites in birds.

Example: Nematoparataenia.

Order 9. Nippotaeniidea

1. No scolex but a well-defined terminal sucker.
2. Proglottids few.Vitellaria few.
3. Parasites in freshwater fishes of japan,

Example:Nippotaenia, Amurotaenia.

Order 10. Caryophyllidea

1. Scolex without true sucker or bothria.
2. Eggs non-embryonated when laid.
3. parasitic in fishes.

Example: caryophyllene. Archives.

Order 11. Spathebothridea

1. Scolex without suckers or brothers.
2. Tests are medullary.The ovary is medium.
3. Parasites in primitives fishes.

Example: Spathebothrium.

Reference:

Aggarwal Sarita. A Text Book of Biology, New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

http://www.parasitesinhumans.org/

https://web.stanford.edu/class/

https://en.m.wikipedia.org/wiki/Flatworm