Earthworm — Core Principles
Core Principles
Earthworms are segmented, terrestrial invertebrates belonging to the phylum Annelida, class Oligochaeta, with *Pheretima posthuma* being a common Indian species. Their cylindrical body is divided into numerous segments (metameres), and they possess a prominent glandular clitellum (segments 14-16) crucial for reproduction.
They are detritivores, feeding on decaying organic matter in moist soil, and are vital for soil aeration, drainage, and nutrient cycling, earning them the title 'farmer's friend'. Internally, they have a complete digestive system with specialized structures like a gizzard for grinding and a typhlosole in the intestine to increase absorption surface.
Respiration occurs cutaneously through their moist skin. They have a closed circulatory system with haemoglobin dissolved in plasma and five pairs of pulsatile hearts. Excretion is carried out by segmentally arranged nephridia (septal, integumentary, pharyngeal).
Earthworms are hermaphrodites but typically undergo cross-fertilization, with development occurring directly within a cocoon secreted by the clitellum. Their nervous system includes a cerebral ganglion and a ventral nerve cord.
Understanding these basic features is fundamental for NEET preparation.
Important Differences
vs Cockroach
| Aspect | This Topic | Cockroach |
|---|---|---|
| Phylum | Annelida | Arthropoda |
| Body Segmentation | Metamerically segmented (similar segments) | Body divided into head, thorax, abdomen (heteronomous segmentation) |
| Circulatory System | Closed (blood in vessels, haemoglobin in plasma) | Open (hemolymph in hemocoel, no respiratory pigment) |
| Respiratory System | Cutaneous respiration (through moist skin) | Tracheal system (spiracles and tracheae) |
| Excretory System | Nephridia (segmentally arranged) | Malpighian tubules |
| Reproduction | Hermaphrodite, cross-fertilization, direct development in cocoon | Dioecious, internal fertilization, indirect development (nymphal stages) |
| Locomotion | Peristaltic movement using setae and body wall muscles | Walking (legs), flying (wings) |
| Ecological Role | Detritivore, soil aerator, 'farmer's friend' | Scavenger, pest, vector for diseases |