🪱 Earthworm – Morphology and Anatomy
Earthworms are classic examples to understand organ system level of organisation in animals. They belong to:
- Phylum: Annelida
- Class: Oligochaeta
- Common Indian species: Pheretima posthuma
They are terrestrial, burrowing, hermaphrodite animals found in moist, humus-rich soil. Earthworms are called “friends of farmers” because they improve soil structure and fertility.
1️⃣ Morphology (External Features)
Body Shape and Segmentation (Metamerism)
- Body is long, cylindrical, soft, and segmented.
- Segments are called metameres (repeated units) → helps in efficient movement.
- Usually about 100–120 segments.
- Bilateral symmetry (right and left halves are similar).
Dorsal vs Ventral Surface
- Dorsal side: darker (more pigment)
- Ventral side: lighter and bears setae more clearly
Prostomium and Peristomium
- Prostomium: a small, tongue-like sensory lobe in front of the mouth.
- Helps in touch and digging.
- Peristomium (1st segment): contains the mouth.
Setae (for Locomotion)
- Tiny bristle-like structures on most segments (except first, last, and clitellar segments).
- They help in anchoring and movement through soil.
Clitellum (Reproductive Band)
- A thick, glandular, saddle-shaped band on segments 14–16.
- Very important because it forms the cocoon during reproduction.
Important External Openings
| Opening | Segment(s) | What it does |
|---|---|---|
| Mouth | 1st segment (peristomium) | Takes in soil + organic matter |
| Anus | Last segment | Removes undigested waste |
| Female genital pore | 14th segment | Releases ova |
| Male genital pore | 18th segment | Releases sperm |
| Nephridiopores | Many segments (body wall) | Remove nitrogenous wastes |
2️⃣ Digestive System (Alimentary Canal)
Earthworm is a detritivore (feeds on decaying organic matter). The alimentary canal is a straight tube from mouth to anus.
Step-by-step digestion idea
Mouth → Grinding → Neutralisation → Absorption → Egestion
| Part | Segment region | Main role |
|---|---|---|
| Buccal cavity + Pharynx | 1–4 | Ingestion + swallowing |
| Oesophagus | 5–7 | Passage of food |
| Gizzard | 8–9 | Grinding food (like a crusher) |
| Stomach | 9–14 | Digestion; calciferous glands help neutralise acids |
| Intestine | 15–last | Digestion + absorption |
| Anus | Last | Egestion |
Typhlosole
- A dorsal internal fold in the intestine.
- It increases surface area → more absorption.
3️⃣ Circulatory System (Closed Type)
Earthworm has a closed circulatory system:
- Blood flows only inside blood vessels (not in open spaces).
Key points to remember
- Blood is red because haemoglobin is dissolved in plasma (not inside RBC like humans).
- Main vessels:
- Dorsal vessel: carries blood forward (anterior direction)
- Ventral vessel: carries blood backward (posterior direction)
Hearts (Aortic arches)
- Earthworm has 4 pairs of hearts in the region of segments 7–11.
- They pump blood and maintain circulation.
4️⃣ Respiration
Earthworm has no special respiratory organs.
- Respiration occurs through moist skin by diffusion.
- इसलिए earthworms are found in moist soil; dry skin = poor gas exchange.
5️⃣ Excretory System (Nephridia)
The excretory organs are nephridia. They help in:
- Removal of nitrogenous wastes
- Osmoregulation (maintaining water and salt balance)
Types of Nephridia
- Pharyngeal nephridia – in segments 4, 5, 6
- Septal nephridia – on the intersegmental septa
- Integumentary nephridia – on the body wall
Note: Earthworm does not have Malpighian tubules (those are in insects).
6️⃣ Nervous System
Earthworm shows a well-developed nervous system:
- Nerve ring around pharynx
- Ventral nerve cord running throughout the body
Brain region
- Cerebral ganglia (brain) lies above the pharynx.
- Connected to the ventral nerve cord by circum-pharyngeal connectives.
7️⃣ Reproductive System (Hermaphrodite + Cross Fertilisation)
Earthworm is hermaphrodite (male + female organs in the same body), but it usually shows cross-fertilisation.
Major organs and segments
| Organ | Segment(s) | Function |
|---|---|---|
| Testes | 10 & 11 | Produce sperms |
| Seminal vesicles | 11–12 region (functionally storage/maturation) | Store/mature sperms |
| Ovaries | 13 | Produce ova |
| Female genital pore | 14 | Release ova |
| Male genital pore | 18 | Release sperms |
How fertilisation happens (easy story)
- Two earthworms exchange sperms.
- Clitellum secretes mucus + forms a cocoon.
- Eggs + received sperms enter the cocoon.
- Fertilisation occurs inside the cocoon (outside the body).
8️⃣ Economic Importance (Why Farmers Love Earthworm)
- Burrowing makes soil porous → better aeration and water penetration.
- Castings increase humus and mineral availability.
- Used in vermicomposting → organic manure.
- Also act as food for birds/reptiles (food chain role).
💡 Questions and Answers
Q1. Why is earthworm called the “friend of farmers”?
A1.
Earthworms keep the soil healthy. When they burrow, they make the soil loose and airy, so roots get oxygen easily. Their castings add humus and nutrients, which improves fertility. That’s why farmers benefit a lot.
Q2. What is metamerism in earthworm?
A2.
Metamerism means the body is divided into many similar ring-like segments. In earthworm, these segments help in better movement and make body functions more efficient.
Q3. What is the function of setae?
A3.
Setae are tiny bristles that help the earthworm grip the soil. They act like anchors so the worm can crawl and burrow easily.
Q4. What is the role of the prostomium?
A4.
Prostomium is a small lobe in front of the mouth. It works as a sensory organ and helps the earthworm in digging and detecting the surroundings.
Q5. Why does earthworm need moist skin?
A5.
Earthworms breathe through their skin. If the skin is dry, oxygen cannot dissolve and diffuse inside. So they require moist skin for respiration.
Q6. Why is the circulatory system called “closed” in earthworm?
A6.
Because blood flows only inside blood vessels (dorsal vessel, ventral vessel, hearts). It does not directly bathe organs in open spaces.
Q7. What is the function of the gizzard?
A7.
The gizzard acts like a grinding chamber. Since earthworms eat soil with organic matter, the gizzard crushes it into fine particles for digestion.
Q8. What are calciferous glands and why are they important?
A8.
Calciferous glands are present in the stomach region. They help to neutralize the acids (like humic acids) present in soil, so digestion becomes easier.
Q9. What is typhlosole and what is the function of the typhlosole in earthworm?
A9.
Typhlosole is an internal fold of the intestine that increases the surface area, so more nutrients can be absorbed efficiently.
Q10. Why is earthworm called hermaphrodite but still shows cross-fertilisation?
A10.
Even though one earthworm has both male and female organs, it usually mates with another earthworm and exchanges sperms. Later, fertilisation occurs in the cocoon. So it is hermaphrodite but prefers cross-fertilisation.
Q11. What are nephridia?
A11.
Nephridia are the excretory organs of earthworm. They remove wastes and also maintain water and salt balance (osmoregulation).
Q12. Where is the clitellum present and what does it do?
A12.
Clitellum is present on segments 14–16. It secretes mucus and forms a cocoon, inside which fertilisation and early development occur.
Q13. Why does fertilization in earthworm occur externally
A13.
Earthworms exchange sperms during mating. Later, the clitellum forms a cocoon outside the body. Eggs and sperms enter this cocoon, and fertilisation occurs in the cocoon (outside the body).