Unit 2: Platyhelminthes, Nemathelmithes and Annelida

Table of Contents

1. Phylum Platyhelminthes (Flatworms)

Platyhelminthes (from Greek: platys = flat, helmins = worm) are dorsoventrally flattened, triploblastic, acoelomate animals. They exhibit an organ-system grade of organization.

1.1 General Characters and Classification

General Characters

Classification of Platyhelminthes (Up to Classes)

Class Lifestyle Key Characteristics Examples
Turbellaria Mostly free-living Body covered with cilia. Mouth is ventral. Planaria (Dugesia)
Trematoda Parasitic (Flukes) Body unsegmented and leaf-like. Possess oral and ventral suckers for attachment. Fasciola (liver fluke)
Cestoda Endoparasitic (Tapeworms) Body is long, ribbon-like, and divided into a scolex, neck, and proglottids. Digestive system is absent. Taenia solium (pork tapeworm)

1.2 Life History of Taenia solium (Pork Tapeworm)

Taenia solium is a digenetic parasite, meaning it requires two hosts to complete its life cycle.

  • Primary Host: Human (where the adult worm lives in the small intestine).
  • Intermediate Host: Pig (where the larval stage develops).

Life Cycle

  1. An infected human passes gravid (egg-filled) proglottids in their feces.
  2. The eggs (oncospheres) are ingested by a pig through contaminated food or water.
  3. In the pig's intestine, the oncospheres hatch, penetrate the intestinal wall, and enter the bloodstream.
  4. They travel to striated muscles (tongue, neck, shoulder) and develop into the larval stage called Cysticercus cellulosae (also known as a bladder worm).
  5. A human becomes infected by eating raw or undercooked pork containing these cysticerci.
  6. In the human intestine, the cysticercus wall is digested, the scolex evaginates and attaches to the intestinal wall, and the neck starts budding off new proglottids, growing into an adult tapeworm.
Pathogenicity (Disease):
  • Taeniasis: Caused by the adult worm in the intestine. Symptoms are usually mild (abdominal pain, nausea).
  • Cysticercosis: A much more dangerous condition caused by the accidental ingestion of eggs by humans (making the human an intermediate host). The larvae migrate to various tissues, including the brain (neurocysticercosis), muscles, and eyes, forming cysts that can cause seizures and neurological damage.

2. Phylum Nemathelminthes (Roundworms)

Nematodes are cylindrical, unsegmented, triploblastic, pseudocoelomate worms. They are one of the most abundant animal groups on Earth.

2.1 General Characters and Classification

General Characters

  • Body Form: Cylindrical, tapering at both ends.
  • Symmetry: Bilateral.
  • Body Wall: Covered by a tough, flexible, non-living cuticle, which is molted periodically.
  • Body Cavity: Pseudocoelomate (a false coelom not fully lined by mesoderm).
  • Digestive System: Complete, with a distinct mouth and anus (a "tube-within-a-tube" body plan).
  • Circulatory & Respiratory Systems: Absent.
  • Reproduction: Dioecious (sexes are separate), with sexual dimorphism (males are usually smaller than females).

Classification of Nemathelminthes (Up to Classes)

Class Key Characteristics Examples
Aphasmidia (Adenophorea) Phasmids (posterior sensory organs) are absent. Excretory system is simple or absent. Trichinella, Trichuris
Phasmidia (Secernentea) Phasmids are present. Excretory system is well-developed with lateral canals. Most parasitic nematodes belong here. Ascaris, Wuchereria, Ancylostoma (hookworm)

2.2 Life History of Ascaris lumbricoides (Giant Roundworm)

Ascaris lumbricoides is a monogenetic parasite (completes its life cycle in a single host, the human).

Life Cycle

  1. An infected person passes fertilized eggs in their feces.
  2. The eggs become infective after developing into an embryonated stage in moist soil.
  3. Infection occurs when a human ingests these embryonated eggs through contaminated food or water.
  4. The eggs hatch in the small intestine, and the larvae penetrate the intestinal wall to enter the bloodstream.
  5. They undertake a remarkable migration: blood → liver → heart → lungs.
  6. In the lungs, they break into the alveoli, crawl up the trachea, and are swallowed back into the intestine.
  7. In the intestine, they mature into adult worms, mate, and produce eggs, continuing the cycle.
Pathogenicity (Ascariasis): The disease caused by the infection. Larval migration can cause pneumonia-like symptoms (Loeffler's syndrome). A heavy worm load in the intestine can cause abdominal pain, malnutrition, and even intestinal blockage.

3. Phylum Annelida (Segmented Worms)

Annelids (from Latin: annellus = little ring) are triploblastic, coelomate animals characterized by true metameric segmentation.

3.1 General Characters and Classification

General Characters

  • Habitat: Found in marine, freshwater, and moist terrestrial environments.
  • Segmentation: Body exhibits true metamerism (serial repetition of segments), both externally and internally.
  • Body Wall: Covered by a thin cuticle, with circular and longitudinal muscles.
  • Locomotory Structures: Possess chitinous setae or parapodia.
  • Body Cavity: True coelomates (schizocoelom). The coelom is well-developed and partitioned by septa.
  • Circulatory System: Closed type, with blood contained within vessels.
  • Excretory System: Consists of segmentally arranged coiled tubes called nephridia.

Classification of Annelida (Up to Classes)

Class Key Characteristics Examples
Polychaeta Mostly marine. Have numerous setae arranged on lateral appendages called parapodia. Sexes are separate. Nereis (clam worm)
Oligochaeta Mostly terrestrial and freshwater. Setae are few. Parapodia are absent. Hermaphroditic, with a clitellum for cocoon formation. Pheretima (earthworm)
Hirudinea Mostly freshwater ectoparasites. Body is dorsoventrally flattened. Lack setae and parapodia. Have anterior and posterior suckers. Hirudinaria (leech)

3.2 Digestive System of Leech

Leeches (Class Hirudinea, e.g., Hirudinaria) are primarily sanguivorous (blood-sucking). Their digestive system is highly adapted for this diet.

  1. Mouth and Pharynx: The leech attaches to a host using its posterior sucker. The anterior sucker surrounds the triradiate (Y-shaped) mouth, which contains three chitinous jaws. The jaws make an incision, and the muscular pharynx sucks in the blood.
  2. Anticoagulation: Salivary glands secrete hirudin, a powerful anticoagulant that prevents the host's blood from clotting, ensuring a steady flow. Saliva also contains an anesthetic.
  3. Crop: The largest part of the alimentary canal. It is a long tube with 10 pairs of lateral pouches called diverticula. The ingested blood is stored in this massive crop. This allows the leech to store many times its own body weight in blood.
  4. Stomach and Intestine: Digestion is a very slow process, carried out by enzymes (reportedly from symbiotic bacteria) as the blood moves from the crop into the small stomach and then the intestine.
  5. Storage: A single large meal can sustain a leech for several months due to the slow digestion and large storage capacity of the crop.