Anatomy — Revision Notes

⚡ 30-Second Revision

Body Wall: — Epidermis, circular & longitudinal muscles, coelom.
Digestive: — Mouth $\rightarrow$ Buccal Cavity $\rightarrow$ Pharynx $\rightarrow$ Oesophagus (Calciferous Glands) $\rightarrow$ Gizzard $\rightarrow$ Intestine (Typhlosole) $\rightarrow$ Anus.
Circulatory: — Closed type. Dorsal vessel (anterior flow), Ventral vessel (posterior flow), Aortic arches (hearts in segments 7, 9, 10, 11 in *Pheretima*). Haemoglobin in plasma.
Respiration: — Cutaneous (through moist skin).
Excretory (Nephridia): — Septal (segments 15-last, enteronephric), Integumentary (segments 3-last, exonephric), Pharyngeal (segments 4, 5, 6, enteronephric).
Nervous: — Supra-pharyngeal ganglia (brain), circum-pharyngeal connectives, Ventral nerve cord with segmental ganglia.
Reproductive (Hermaphrodite):

- Male: Testes (segments 10, 11), Seminal Vesicles (segments 11, 12), Vasa deferentia, Male genital pores (segment 18). - Female: Ovaries (segment 13), Oviducts, Female genital pore (segment 14), Spermathecae (segments 6, 7, 8, 9).

Clitellum: — Segments 14-16, forms cocoon.

2-Minute Revision

Earthworm anatomy is characterized by its segmented body and well-developed organ systems. The body wall comprises an epidermis, circular and longitudinal muscles, and a coelom. The digestive system is a straight tube: mouth, buccal cavity, pharynx, oesophagus (with calciferous glands to neutralize acids), gizzard (for grinding), and a long intestine with a typhlosole to increase absorption.

Undigested matter exits via the anus. Its circulatory system is closed, featuring a dorsal blood vessel, a ventral blood vessel, and pulsatile aortic arches (hearts) in specific anterior segments. Blood contains dissolved haemoglobin.

Respiration occurs entirely through the moist skin (cutaneous respiration). Excretion is managed by three types of nephridia: septal (from segment 15, enteronephric), integumentary (from segment 3, exonephric), and pharyngeal (segments 4-6, enteronephric).

The nervous system includes a 'brain' (supra-pharyngeal ganglia) connected to a ventral nerve cord with segmental ganglia. Earthworms are hermaphrodites, possessing both male (testes, seminal vesicles, male pores in segment 18) and female (ovaries, oviducts, female pore in segment 14, spermathecae) reproductive organs.

The clitellum (segments 14-16) is crucial for cocoon formation, where fertilization and development occur.

5-Minute Revision

A comprehensive review of earthworm anatomy begins with its fundamental body plan: a metamerically segmented, coelomate structure. The body wall consists of a protective cuticle, a single-layered epidermis, and antagonistic circular and longitudinal muscle layers, facilitating peristaltic locomotion. The coelom, a true body cavity, acts as a hydrostatic skeleton.

The digestive system is a complete, straight tube. Food enters through the mouth, passes through the buccal cavity and muscular pharynx, then the oesophagus (where calciferous glands neutralize humic acids).

The muscular gizzard grinds food, which then moves to the long intestine. The intestine's dorsal wall features a prominent fold, the typhlosole, significantly increasing the absorptive surface area.

Digestion and absorption are completed here, with undigested material expelled as castings.

The circulatory system is closed. A contractile dorsal blood vessel pumps blood anteriorly, while a non-contractile ventral blood vessel carries it posteriorly. These are connected by pulsatile aortic arches (hearts) in segments 7, 9, 10, and 11 (in *Pheretima*). Blood contains haemoglobin dissolved in the plasma, giving it a red color and enabling efficient oxygen transport.

Respiration is cutaneous; oxygen diffuses across the moist, permeable skin into epidermal capillaries, and carbon dioxide diffuses out. The skin's moisture, maintained by mucus, is vital.

Excretion is performed by segmentally arranged nephridia. There are three types: septal nephridia (segments 15-last), which are enteronephric (discharge into intestine); integumentary nephridia (segments 3-last), which are exonephric (open to exterior); and pharyngeal nephridia (segments 4, 5, 6), also enteronephric. All filter coelomic fluid and blood to remove nitrogenous waste.

The nervous system is centralized, featuring a 'brain' (supra-pharyngeal ganglia) in segment 3, connected by circum-pharyngeal connectives to a ventral nerve cord. This cord runs posteriorly, bearing segmental ganglia in each segment.

Earthworms are hermaphrodites, possessing both male and female reproductive organs. Male organs include two pairs of testes (segments 10, 11) within testis sacs, two pairs of seminal vesicles (segments 11, 12) for sperm maturation, vasa deferentia, and a pair of male genital pores on segment 18.

Female organs include a pair of ovaries (segment 13), oviducts leading to a single female genital pore on segment 14, and four pairs of spermathecae (segments 6, 7, 8, 9) for storing received sperm.

The clitellum (segments 14-16) is a glandular band that secretes mucus for copulation, albumen for embryo nourishment, and the cocoon wall, where fertilization and development occur after cross-fertilization.

Prelims Revision Notes

1
Body Wall: — Epidermis (single layer, cuticle, mucus glands), Circular muscles (outer), Longitudinal muscles (inner). Coelom (true, segmented, hydrostatic skeleton).
2
Digestive System:

* Mouth (segment 1, covered by prostomium). * Buccal Cavity (segments 1-3). * Pharynx (segments 3-4, muscular, glandular). * Oesophagus (segments 5-7, contains calciferous glands in 11-13 in *Pheretima* to neutralize humic acid).

* Gizzard (segments 8-9 in *Pheretima*, muscular, grinds food). * Stomach (segments 9-14 in *Pheretima*). * Intestine (segment 15 to last, main digestion/absorption). Typhlosole (dorsal fold from segment 26, increases surface area).

Intestinal caeca (segment 26 in *Pheretima*). * Anus (last segment).

1
Circulatory System: — Closed type.

* Dorsal Blood Vessel: Contractile, pumps blood anteriorly. * Ventral Blood Vessel: Non-contractile, carries blood posteriorly. * Aortic Arches (Hearts): Pulsatile, connect dorsal & ventral vessels (e.g., 2 pairs in segments 7 & 9, and 2 pairs of anterior loops in 10 & 11 in *Pheretima*). * Blood: Red, haemoglobin dissolved in plasma.

1
Respiration: — Cutaneous (through moist skin), no specialized organs.
2
Excretory System (Nephridia):

* Septal Nephridia: Segments 15 to last, attached to septa, enteronephric (discharge into intestine). * Integumentary Nephridia: Segments 3 to last, scattered in body wall, exonephric (open to exterior), lack nephrostome. * Pharyngeal Nephridia: 3 paired tufts in segments 4, 5, 6, enteronephric (discharge into buccal cavity/pharynx).

1
Nervous System:

* Brain: Supra-pharyngeal ganglia (segment 3, dorsal). * Circum-pharyngeal Connectives: Connect brain to sub-pharyngeal ganglia. * Sub-pharyngeal Ganglia: (segment 3, ventral). * Ventral Nerve Cord: Double, solid, runs from segment 4 posteriorly, with segmental ganglia.

1
Reproductive System: — Hermaphrodite (cross-fertilization).

* Male: Testes (2 pairs, segments 10, 11 in testis sacs), Seminal Vesicles (2 pairs, segments 11, 12, store sperm), Vasa deferentia, Accessory glands (segments 17, 19), Male genital pores (1 pair, segment 18, ventrolateral). * Female: Ovaries (1 pair, segment 13), Oviducts, Female genital pore (1, mid-ventral, segment 14), Spermathecae (4 pairs, segments 6, 7, 8, 9, store received sperm). * Clitellum: Glandular band, segments 14-16, secretes mucus, albumen, cocoon wall.

Vyyuha Quick Recall

To remember the order of digestive organs: My Brother Plays On Green Islands All Night.

Mouth
Buccal cavity
Pharynx
Oesophagus
Gizzard
Intestine
Anus

For Nephridia Types and Openings (Pheretima): Septal Into Intestine (Septal $\rightarrow$ Enteronephric) Integumentary Outside (Integumentary $\rightarrow$ Exonephric) Pharyngeal Into Pharynx (Pharyngeal $\rightarrow$ Enteronephric)