Q.What are earthworm casts?

Pheretima (the Indian earthworm) has a complete, one-way alimentary canal running the length of its body, specialised region by region for ingestion, grinding, chemical digestion, absorption and egestion of soil-derived organic matter.

Regions of the alimentary canal (front to back):

1. Buccal cavity (segments 1–3): a small chamber just behind the mouth that helps draw in soil and food particles.
2. Pharynx (segments 4–5): muscular, and acts as a suction pump, aided by pharyngeal glands that secrete mucus and digestive (proteolytic) enzymes to lubricate and begin digestion of the food.
3. Oesophagus (segments 6–7 to about 13/14): a narrow tube connecting the pharynx to the crop/gizzard; it bears paired calciferous glands (of Morren) in some segments, which secrete calcium carbonate to neutralise excess humic/carbonic acids present in the ingested soil.
4. Crop (segment 14–15): a thin-walled sac that temporarily stores food.
5. Gizzard (segment 16–17): a thick, muscular chamber lined with a tough cuticle that grinds and triturates the soil and organic matter into fine particles, aided by soil grit acting like millstones.
6. Stomach–intestine (from segment 18 to near the last segment): a long tube where the bulk of chemical digestion and absorption occurs. Its dorsal wall is folded inward to form the typhlosole, a ridge that greatly increases the internal absorptive/secretory surface area without increasing the worm's girth.
7. Anus: the terminal opening at the last segment through which undigested soil is egested.

Process of digestion:

• The worm ingests soil rich in organic/decaying matter through the mouth and buccal cavity, aided by pharyngeal suction.
• Mucus and enzymes from the pharyngeal glands begin softening and lubricating the food as it passes down the oesophagus.
• Calciferous glands neutralise acidity in the soil, keeping the gut environment favourable for enzyme activity.
• The muscular gizzard grinds the soil/food into a fine paste using peristaltic and grinding action. …