Earthworm

🪱 Earthworm – Class 11 Biology NEB | Complete Notes, Anatomy, and Functions The earthworm (Pheretima posthuma) is an important invertebrate organism covered in the Class 11 Biology NEB curriculum under the chapter on Annelida. Earthworms play a key role in soil aeration and fertility and are commonly known as the "farmer's friend" due to their ecological benefits. 📘 Scientific Classification: Phylum: Annelida Class: Oligochaeta Genus: Pheretima Species: posthuma 🔍 General Characteristics: Long, cylindrical, and segmented body Lives in moist soil (terrestrial and burrowing) Lacks a skeleton but has a hydrostatic skeleton Moves using setae (tiny bristles on each segment) Hermaphrodite (both male and female reproductive organs present) 🧠 Body Systems Overview: ✅ 1. Digestive System: Complete and well-developed Food path: Mouth → Buccal cavity → Pharynx → Oesophagus → Gizzard → Intestine → Anus Gizzard helps in grinding food Digestion and absorption occur in the intestine ✅ 2. Circulatory System: Closed circulatory system Blood vessels: Dorsal, ventral, and lateral hearts (5 pairs of hearts) Blood contains haemoglobin dissolved in plasma ✅ 3. Respiratory System: No lungs or gills Gaseous exchange occurs through moist skin ✅ 4. Nervous System: A simple system with a nerve ring and ventral nerve cord Pair of cerebral ganglia acts as a primitive brain ✅ 5. Excretory System: Excretes nitrogenous waste (mainly urea and ammonia) Excretory organs: Nephridia ✅ 6. Reproductive System: Hermaphroditic Cross-fertilization occurs during mating Reproduction via cocoon formation from the clitellum 🌱 Economic & Ecological Importance: Improve soil structure and fertility Decompose organic matter Used in vermicomposting Indicate soil health 🎯 Quick Facts: Has about 100–120 segments Clitellum present from segments 14 to 16 No specialized sense organs, but sensitive to light and touch 📌 Why Study Earthworm in Class 11 NEB Biology? Understanding earthworms helps students grasp key biological concepts like segmentation, coelom, closed circulation, and reproductive adaptations in lower organisms. It's also crucial for understanding the role of invertebrates in ecosystem sustainability.