Earthworm — Revision Notes

⚡ 30-Second Revision

Phylum: — Annelida, Class: Oligochaeta (*Pheretima posthuma*)
Body: — Segmented (metameric), cylindrical, reddish-brown.
Clitellum: — Glandular band, segments 14-16, secretes cocoon.
Setae: — S-shaped chitinous bristles for locomotion.
Mouth: — 1st segment (peristomium), Prostomium (overhangs).
Anus: — Terminal, posterior.
Male Genital Pores: — Pair on 18th segment.
Female Genital Pore: — Single on 14th segment.
Spermathecal Pores: — 4 pairs, intersegmental grooves of 5/6, 6/7, 7/8, 8/9.
Digestive System: — Complete. Mouth $\rightarrow$ Pharynx $\rightarrow$ Oesophagus $\rightarrow$ Gizzard (grinds) $\rightarrow$ Stomach (calciferous glands neutralize acid) $\rightarrow$ Intestine (typhlosole increases absorption) $\rightarrow$ Anus.
Circulatory System: — Closed type. Haemoglobin dissolved in plasma. 5 pairs of hearts (lateral hearts in 7, 9; latero-oesophageal hearts in 12, 13).
Respiration: — Cutaneous (through moist skin).
Excretion: — Nephridia (septal, integumentary, pharyngeal). Septal & Pharyngeal are enteronephric; Integumentary are exonephric.
Nervous System: — Cerebral ganglia (brain) + Ventral nerve cord.
Reproduction: — Hermaphrodite, cross-fertilization, direct development (in cocoon).
Ecological Role: — 'Farmer's friend', vermicomposting, soil fertility.

2-Minute Revision

The earthworm (*Pheretima posthuma*) is a segmented annelid, vital for soil health. Its body is metamerically segmented, with a prominent clitellum (segments 14-16) crucial for reproduction. Locomotion is aided by setae and peristaltic contractions.

The digestive system is a complete tube, featuring a muscular gizzard for grinding food, calciferous glands in the stomach to neutralize humic acid, and an intestinal typhlosole to maximize nutrient absorption.

Respiration occurs cutaneously through its moist skin, necessitating a damp environment. It possesses a closed circulatory system with haemoglobin dissolved in the plasma and five pairs of pulsatile hearts.

Excretion is handled by segmentally arranged nephridia, categorized as septal, integumentary, and pharyngeal, with some being enteronephric (opening into the gut) and others exonephric (opening to the exterior).

Earthworms are hermaphrodites but typically undergo cross-fertilization, with direct development occurring within a cocoon. Their ecological role as 'farmer's friends' through soil aeration, nutrient cycling, and vermicomposting is highly significant for NEET.

5-Minute Revision

For NEET, a deep dive into the earthworm's structural organization is essential. Remember its classification: Phylum Annelida, Class Oligochaeta, with *Pheretima posthuma* being the key species. The body is metamerically segmented, typically 100-120 segments.

The clitellum, a glandular band (segments 14-16), is non-segmented and critical for cocoon formation. External apertures include the mouth (1st segment), anus (terminal), male genital pores (18th segment), female genital pore (14th segment), and four pairs of spermathecal pores (intersegmental grooves of 5/6, 6/7, 7/8, 8/9).

Setae provide grip for locomotion.

Internally, the digestive system is a straight tube: buccal cavity $\rightarrow$ pharynx $\rightarrow$ oesophagus $\rightarrow$ gizzard (grinds food) $\rightarrow$ stomach (calciferous glands neutralize humic acid) $\rightarrow$ intestine (typhlosole increases absorption surface) $\rightarrow$ anus.

Respiration is cutaneous, requiring moist skin. The circulatory system is closed, with haemoglobin in plasma, and five pairs of 'hearts' (lateral and latero-oesophageal) pump blood. Excretion is via nephridia: septal (enteronephric, from segment 15), integumentary (exonephric, from segment 3), and pharyngeal (enteronephric, segments 4, 5, 6).

The nervous system comprises cerebral ganglia and a ventral nerve cord. Reproduction is hermaphroditic, but cross-fertilization is the norm, with direct development inside a cocoon. Its ecological role as a 'farmer's friend' through soil aeration, nutrient cycling, and vermicomposting is a frequently tested concept.

Pay close attention to segment numbers and specific functions of each organ.

Prelims Revision Notes

Earthworm (*Pheretima posthuma*) - NEET Revision Notes

1. Classification:

Phylum: Annelida
Class: Oligochaeta
Genus: *Pheretima*, Species: *posthuma*

2. External Morphology:

Body: — Cylindrical, reddish-brown, metamerically segmented (100-120 segments).
Prostomium: — Fleshy lobe overhanging mouth (not a segment).
Peristomium: — First segment, surrounds mouth.
Clitellum: — Glandular, non-segmented band, segments 14-16. Secretes cocoon.
Setae: — S-shaped, chitinous bristles, absent in 1st, last, and clitellar segments. Aid locomotion.
Apertures:

* Mouth: Ventral, 1st segment. * Anus: Terminal. * Male Genital Pores: Pair, ventrolateral, 18th segment. * Female Genital Pore: Single, median, ventral, 14th segment. * Spermathecal Pores: 4 pairs, ventrolateral, intersegmental grooves of 5/6, 6/7, 7/8, 8/9. * Nephridiopores: Numerous, scattered.

3. Digestive System:

Complete, straight tube.
Mouth — $\rightarrow$ Buccal cavity (1-3) $\rightarrow$ Pharynx (3-4, pharyngeal glands) $\rightarrow$ Oesophagus (5-7) $\rightarrow$ Gizzard (8-9, grinds food) $\rightarrow$ Stomach (9-14, calciferous glands neutralize humic acid) $\rightarrow$ Intestine (15-anus, typhlosole from 26th segment increases absorption) $\rightarrow$ Anus.

4. Circulatory System:

Closed type. — Blood flows in vessels.
Blood: — Red, due to haemoglobin dissolved in plasma. Contains amoeboid corpuscles.
Hearts (Pulsatile vessels): — 5 pairs.

* Lateral hearts: 7th and 9th segments. * Latero-oesophageal hearts: 12th and 13th segments.

5. Respiratory System:

No specialized organs.
Cutaneous respiration: — Gaseous exchange through moist, vascularized skin.

6. Excretory System:

Nephridia: — Segmentally arranged, coiled tubules.

* Septal Nephridia: From 15th segment to last. Enteronephric (open into intestine). * Integumentary Nephridia: From 3rd segment to last. Exonephric (open to exterior). * Pharyngeal Nephridia: 3 paired tufts in segments 4, 5, 6. Enteronephric (open into pharynx/buccal cavity).

7. Nervous System:

Cerebral ganglia (brain): — Dorsal, 3rd segment.
Ventral nerve cord: — Double, solid, runs posteriorly.
Sensory receptors: — Chemoreceptors, photoreceptors (no eyes).

8. Reproductive System:

Hermaphrodite (Monoecious): — Both male and female organs in one individual.
Cross-fertilization: — Predominant, prevents self-fertilization (protandry).
Male Organs:

* Testes: 2 pairs, 10th & 11th segments. * Seminal Vesicles: 2 pairs, 11th & 12th segments (sperm maturation/storage). * Vas deferens, Prostate glands, Accessory glands.

Female Organs:

* Ovaries: 1 pair, 13th segment. * Oviducts: Open at 14th segment. * Spermathecae (seminal receptacles): 4 pairs, segments 6, 7, 8, 9 (store received sperm).

Development: — Direct (no larval stage), occurs in cocoon.

9. Ecological Importance:

'Farmer's friend': Soil aeration, drainage, nutrient cycling.
Vermi-composting: Converts organic waste into nutrient-rich compost.

Vyyuha Quick Recall

To remember the segment numbers for key reproductive structures in *Pheretima*:

'My Friend Studies Chemistry Twice Over' (for Male, Female, Spermathecal, Clitellum, Testes, Ovaries)

Male Pores: 18
Female Pore: 14
Spermathecal Pores: 5/6, 6/7, 7/8, 8/9
Clitellum: 14-16
Testes: 10, 11
Ovaries: 13