🪸 Introduction

Phylum Aschelminthes (Roundworms): The body of the aschelminthes is circular in cross-section, hence, they are named roundworms.

  • Digestive System: The alimentary canal is complete, with a well-developed muscular pharynx. This represents a significant advancement over Platyhelminthes.

  • Excretion: An excretory tube removes body wastes from the body cavity through an excretory pore.

  • Examples: Ascaris (Roundworm), Wuchereria (Filaria worm - causes elephantiasis), Ancylostoma (Hookworm).

🧬 General Characteristics

  • Habitat: They may be free-living (aquatic and terrestrial) or parasitic in plants and animals.
  • Level of Organisation: They exhibit the organ-system level of body organisation.
  • Symmetry: They are bilaterally symmetrical.
  • Germ Layers: They are triploblastic.
  • Coelom: They are pseudocoelomates (possessing a pseudocoelom, a body cavity not fully lined by mesoderm).

🌊 Reproduction

  • Sexes are separate (dioecious), meaning individuals are either male or female.
  • Often, females are longer than males (sexual dimorphism).
  • Fertilisation is internal.
  • Development may be direct (young ones resemble the adult) or indirect (involving larval stages).

💡 Questions and Answers

Q1. What is the difference between direct and indirect development?

A1.

  • Direct Development: The young organism hatches or is born resembling a miniature adult. There is no distinct larval stage. Example: Humans, many reptiles.
  • Indirect Development: The organism undergoes one or more intermediate larval stages that are morphologically different from the adult. Metamorphosis occurs to transform the larva into the adult. Example: Frogs, butterflies, many Aschelminthes.

Q2. What type of body cavity is found in Aschelminthes? Explain it.

A2. Aschelminthes possess a pseudocoelom. This means they have a body cavity between the body wall and the digestive tract, but unlike a true coelom, it is not completely lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm.

Q3. How does the digestive system of Aschelminthes differ from that of Platyhelminthes?

A3. Aschelminthes have a complete digestive system, meaning it has two openings: a separate mouth for ingestion and an anus for egestion. They also possess a muscular pharynx. Platyhelminthes have an incomplete digestive system with only a single opening (the mouth) serving both functions.

Q4. What does 'dioecious' mean in the context of Aschelminthes reproduction?

A4. 'Dioecious' means that the sexes are separate. In Aschelminthes, there are distinct male and female individuals. This is in contrast to hermaphroditic (monoecious) organisms like Platyhelminthes, where a single individual has both male and female reproductive organs.

Q5. Name two parasitic roundworms and the diseases they cause.

A5.

  1. Wuchereria (Filaria worm): Causes elephantiasis (filariasis), characterized by chronic inflammation and swelling, particularly of the limbs and genitals.
  2. Ascaris (Common roundworm): Causes ascariasis, an intestinal infection.
  3. (Additional) Ancylostoma (Hookworm): Causes intestinal infection and anemia by feeding on blood.