The phylum Platyhelminthes includes flatworms like Planarians, Flukes and Tapeworms. The animals of this phylum are triploblastic acoelomate bilaterians. The evolution of triploblastic condition and bilateral symmetry coincide with the evolution of organs and organ systems, cephalization and centralization of the nervous system. With these features and unidirectional movement, bilaterians have a more active life-style than radially symmetrical animals.
Platyhelminthes is a Greek term. (Gr. Platy=flat, helminth=worm). This term was coined by Gegenbaur. This name indicated the dorso ventrally flattened nature of the body. They have a solid body plan with parenchyma between the gut and the body wall. They also have a very well developed and a complex reproductive system.
General Characters of Phylum Platyhelminthes
- These are mostly parasitic. Some are free-living. The free living forms are chiefly aquatic and the majority are marine forms. A few are terrestrial, confined to humid areas.
- They are triploblastic animals having three primary germ layers viz., ectoderm, endoderm and mesoderm. Mesoderm contributes to the development of true muscle tissue.
- They show bilateral symmetry and cephalization.
- They exhibit organ system of organisation.
- They are acoelomates and lack large fluid filled body cavity. Connective tissue compartments between the gut and the body wall is called parenchyma.
- Gut is a blind sac. Mouth is used for ingestion and egestion. Anus is absent except in some turbellarians. Digestion is both external and internal.
- Respiratory and circulatory system are absent.
- Protonephridia or flame cells are primarily osmoregulatory and secondarily excretory in function. Most of the excretory wastes diffuse out through the body surface.
- Cerebral ganglia constitute the brain. Longitudinal nerve chords are joined by transverse commissures at regular intervals fiving ladder like appearance.
- Sense organs like ocelli and ciliary receptors occur in turbellarians.
- Many turbellarians reproduce asexually by fission or budding.
- They are mostly hermaphrodite. Some are unisexual (Eg: Schistosoma). Fertilization is internal.
- Development is direct or indirect. Life history is simple in free living forms and is complex in parasitic forms. Polyembryony is common in trematodes
Classification of Phylum Platyhelminthes
The phylum Platyhelminthes includes about 20,000 species. This phylum is classified into three classes namely Turbellaria, Trematoda and Cestoda.
The following are the general characters of each of them,
Class I: Turbellaria (L. turbella=stirring)
- This class includes planarians, acoels etc.
- They are usually free living and some forms are also commensals or parasites
- The body of these animals is unsegmented and covered by ciliated epidermis.
- Epidermal glands cells secrete rod-shaped inclusions called rhabdoids. When these inclusions are released to the surface of epidermis they form mucus.
- Adhesive glands help in temporary adhesion to the substratum.
- Mouth is ventral and the pharynx is protrusible.
- The branched gut facilitates the transport of nutrients to all parts of the body. So gut performs the functions as a gastro vascular system
- Planarians have remarkable ability of regeneration. Totipotent cells called as neoblast cells are important in this phenomenon of regeneration.
- Development is direct but larvae like Muller’s larva or Goette’s larva are present in some forms.
Class II: Trematoda (Gr. Trema=hole, eidos=form)
- This class includes flukes
- Generally they live as ectoparasites or endoparasites.
- The body of these animals is segmented and covered by tegument called neodermis.
- Mouth is surrounded by oral sucker and sometimes a ventral sucker or acetabulum is also present.
- The intestine of these animals is bifid
- Life cycle includes miracidium larva, sporocyst, cercaria larva, metacercaria and adult.
Class III: Cestoda (Gr. Kestos=girdle, eidos=form)
- This class includes tapeworm.
- These are ectoparastic in the gut of vertebrates.
- The body of these organisms is divided into scolex, neck and strobila. Body is covered by tegument. The strobila is in turn divided into proglottids.
- They exhibit pseudometamerism.
- Scolex has hooks and suckers for attachment to the gut wall of the host.
- Digestive tract is absent
- The life cycle includes zygote, oncosphere larva, extra intestinal juvenile and intestinal adult.
- Currently the two classes Cestoda and Trematoda are include under the taxon Neodermata. In these flatworms, the body is covered by a non-ciliated syncytial neodermis.
Parasitic adaptations of Phylum Platyhelminthes
Taenia is an endoparasitic worm residing in the internal organs like intestine of humans. To suit its parasitic mode of life it has to overcome to several adverse conditions. And accordingly this tapeworm shows several adjustments. The following are the important parasitic adaptations of Taenia.
- The body is externally covered by tegument which protects against the digestive action of the alkaline digestive juices of the host. This tegument is permeable to water and nutrients.
- The osmotic pressure inside the worm is higher than that of the surrounding host fluid. This helps to permit ready absorption of the digestive food from the host.
- Both the adults and larva lack cilia as there is no need of locomotion.
- A well organized and developed scolex is present with suckers and spines. These suckers and spines help in attachment so that the parasite is not ejected from the host intestine due to peristaltic contractions.
- They are located in the regions where there is continuous supply of pre-digested food material which can be readily absorbed by the parasite. Thus there is no need for alimentary canal. Also to increase the surface area of absorption tegument is modified into microvilli.
- Circulatory, respiratory and sense organs are absent in these parasites.
- Nervous system is also poorly developed as it is not needed.
- Of all the systems, reproductive system is well developed. Enormous number of eggs are produced to overcome the hazards and challenges of survival.
- Resistant shell around the zygotes and embryos gives them protection from unfavorable condition.
- Hermaphroditism and proglottization ensures self-fertilization within the same proglottid or cross fertilization with another proglottid of the same worm.
- Write any two parasitic adaptations of Platyhelminthes.
- How does self fertilization help as an adaptation for parasitism?
- Why do flatworms and tapeworms lack any locomotory organelles?
- The body of trematodes and cestodes is covered by tegument whereas in turbellarians it is covered by epidermis. Give reasons for this.
- Whay is the gut branched in Larger turbellarians?
- Into how many classes is Phylum Platyhelminthes classified.
- What is the main function of flame cells in Phylum Platyhelminthes?
- What does the nervous system of flatworms appear ladder-like?
- Which kind of symmetry is found in flat worms?
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