🪸 Introduction
Phylum Annelida (Segmented Worms): Members of this phylum are characterized by their segmented bodies.
Key Concept: Annelids are the first true coelomates with closed circulation.
Body Wall & Locomotion:
They possess longitudinal and circular muscles which help in locomotion.
Aquatic annelids like Nereis possess lateral appendages called parapodia, which help in swimming.
Terrestrial forms like earthworms move by muscular contractions aided by chitinous setae.
Circulatory System: A closed circulatory system is present (blood flows within vessels). Blood contains respiratory pigments (like haemoglobin in earthworms).
Excretion & Osmoregulation: Excretion occurs through specialized coiled tubules called nephridia (singular: nephridium).
Nervous System: Consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
Examples: Nereis (aquatic, dioecious), Pheretima (Earthworm - terrestrial, monoecious), Hirudinaria (Blood sucking leech - parasitic, monoecious).
🧬 General Characteristics
- Habitat: They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.
- Level of Organisation: They exhibit the organ-system level of body organisation.
- Symmetry: They are bilaterally symmetrical.
- Germ Layers: They are triploblastic.
- Segmentation: A key feature is metameric segmentation. The body surface is distinctly marked out into segments or metameres (Latin, annulus : little ring), and the phylum name Annelida is derived from this.
- Coelom: They are the first major group of coelomate animals, possessing a true coelom.
🌊 Reproduction
- Reproduction is sexual.
- Nereis is dioecious (sexes separate), but earthworms (Pheretima) and leeches (Hirudinaria) are monoecious (hermaphrodites).
- Development can be direct or indirect.
🧠 Summary Comparison Table
| Feature | Platyhelminthes | Aschelminthes | Annelida |
|---|---|---|---|
| Symmetry | Bilateral | Bilateral | Bilateral |
| Germ layers | Triploblastic | Triploblastic | Triploblastic |
| Coelom | Acoelomate | Pseudocoelomate | Coelomate |
| Digestive system | Incomplete | Complete | Complete |
| Circulation | None | None | Closed |
| Excretion | Flame cells | Renette cells | Nephridia |
| Example | Planaria | Ascaris | Earthworm |
💡 Questions and Answers
Q1. Segmentation in the body is first observed in which phylum?
A1. True segmentation, known as metameric segmentation, where the body is distinctly divided into segments both externally and internally with serial repetition of organs, is first observed in Phylum Annelida (e.g., earthworm).
Q2. What is the significance of Annelids being the first coelomates?
A2. The evolution of a true coelom (a body cavity lined entirely by mesoderm) in Annelids was a major evolutionary advancement. The coelom provides space for organ development and complexity, acts as a hydrostatic skeleton aiding locomotion, and allows for better transport of materials within the body. It paved the way for the development of more complex organ systems seen in higher phyla.
Q3. What are parapodia and nephridia? State their functions.
A3.
- Parapodia: These are paired, lateral, fleshy appendages found in aquatic annelids like Nereis. They bear setae and are primarily used for swimming and also help in respiration (gas exchange).
- Nephridia: These are coiled tubular structures found in annelids that function as excretory organs. They collect metabolic waste from the coelomic fluid and body tissues and expel it outside, thus helping in excretion and osmoregulation.
Q4. Differentiate between the circulatory systems of Annelids and Arthropods.
A4.
- Annelids: Possess a closed circulatory system. Blood flows entirely within a network of blood vessels (arteries, veins, capillaries). Respiratory pigments like haemoglobin are often dissolved in the plasma.
- Arthropods: Possess an open circulatory system. Blood (hemolymph) is pumped by the heart into open spaces or sinuses (hemocoel) that directly bathe the tissues, before returning to the heart.
Q5. Give examples of monoecious and dioecious annelids.
A5.
- Monoecious (Hermaphrodite): Pheretima (Earthworm) and Hirudinaria (Leech).
- Dioecious (Sexes Separate): Nereis.