Unit 2: Platyhelminthes, Nemathelminthes and Annelida
1. Platyhelminthes: General Characters and Taenia solium
Platyhelminthes are commonly known as flatworms. They are triploblastic, acoelomate, and bilaterally symmetrical animals.
General Characters
- Dorso-ventrally flattened body.
- Organ-system level of organization.
- Presence of hooks and suckers in parasitic forms for attachment.
- Excretion and osmoregulation occur through specialized cells called flame cells.
Classification
Classification up to classes includes:
- Turbellaria: Mostly free-living (e.g., Planaria).
- Trematoda: Ecto- or endoparasites, commonly called flukes (e.g., Fasciola).
- Cestoda: Endoparasites, commonly called tapeworms (e.g., Taenia).
Life History of Taenia solium
Taenia solium (Pork Tapeworm) is a digenetic parasite.
- Primary Host: Human (where the adult lives in the small intestine).
- Secondary Host: Pig (where larvae develop).
- Infection: Humans are infected by eating "measly pork" containing cysticercus larvae.
2. Nemathelminthes: General Characters and Ascaris lumbricoides
Nemathelminthes (Aschelminthes) are known as roundworms. They are triploblastic, bilaterally symmetrical, and pseudocoelomate.
General Characters
- Long, cylindrical body tapering at both ends.
- The body is covered by a tough, resistant cuticle.
- Alimentary canal is complete with a well-developed muscular pharynx.
- Sexes are separate (dioecious), and females are often longer than males.
Classification
Classified into classes such as Aphasmidia and Phasmidia based on the presence of phasmids (sensory organs).
Life History of Ascaris lumbricoides
Ascaris lumbricoides is a monogenetic parasite (requires only one host).
- Host: Humans.
- Habitat: Small intestine.
- Infection: Occurs by swallowing embryonated eggs through contaminated food or water.
- Migration: Larvae undergo a complex migration through the liver, heart, and lungs before returning to the intestine.
[Image of the life cycle of Ascaris lumbricoides]
3. Annelida: General Characters and Digestive System of Leech
Annelids are true coelomate animals characterized by a segmented body (metamerism).
General Characters
- Triploblastic, bilaterally symmetrical, and truly coelomate.
- Locomotion by setae or parapodia.
- Closed circulatory system (except in leeches).
- Excretion by nephridia.
Classification
Classes include:
- Polychaeta: Many setae, parapodia present (e.g., Nereis).
- Oligochaeta: Few setae, no parapodia (e.g., Pheretima - Earthworm).
- Hirudinea: Fixed number of segments, suckers present (e.g., Hirudinaria - Leech).
Digestive System of Leech
The leech (Hirudinaria granulosa) is a sanguivorous (blood-sucking) animal.
- Mouth: Triradiate opening surrounded by the anterior sucker.
- Pharynx: Muscular, used for sucking blood. It contains salivary glands that secrete hirudin (an anticoagulant).
- Crop: The largest part of the alimentary canal with 10 pairs of diverticula for storing huge quantities of blood.
- Stomach and Intestine: Where slow digestion and absorption occur.
4. Exam Focus Enhancements
Exam Tips
- Flame Cells: Key identifying feature for Platyhelminthes excretion.
- Pseudocoelom: Important character of Nemathelminthes.
- Hirudin: Crucial term to mention when discussing the leech digestive system.
- Tapeworm Life Cycle: Remember it involves two hosts (Human and Pig).
Common Mistakes
- Mixing up primary and secondary hosts for Taenia solium.
- Forgetting that Ascaris larvae migrate through the lungs.
- Confusing the "false" segmentation of tapeworms with the "true" metamerism of annelids.
Frequently Asked Questions
- Define Flame cells and name the phylum where they are found.
- Describe the mode of infection of Taenia solium.
- What is the function of the crop in the leech digestive system?
- Explain the significance of the anticoagulant hirudin.