Leech: digestive, respiratory, circulatory, excretory, nervous and reproductive systems
A leech stores blood in a large crop, breathes through its skin, circulates blood in haemocoelomic channels, excretes by 17 pairs of nephridia and is a hermaphrodite.
Digestive system: the path of blood
The crop is the largest part of the gut. A leech can live for months on one meal of stored blood.
Respiration and circulation
- Respiration: there are no special breathing organs. Gases are exchanged through the skin (cutaneous respiration).
- The skin is kept moist with mucus and has many fine blood capillaries. Oxygen from the water diffuses into the blood, and CO₂ diffuses out.
- Circulation: there are no true blood vessels. Blood flows in haemocoelomic channels, canals of the body cavity filled with a blood-like fluid.
- The fluid is red because haemoglobin is dissolved in its plasma.
- There are four longitudinal channels: one dorsal, one ventral and two lateral. The two lateral channels have muscular walls and inner valves, and act as hearts.
Excretion and the nervous system
- Excretion is by 17 pairs of nephridia, one pair in each segment from the 6th to the 22nd.
- Each nephridium removes wastes from the body fluid. Urine collects in a small vesicle (bladder) and leaves through a nephridiopore on the ventral side.
- Nervous system: a nerve ring around the pharynx, made of the suprapharyngeal ganglion (brain) above, the subpharyngeal ganglion below and two circumpharyngeal connectives at the sides.
- A double ventral nerve cord runs back from the subpharyngeal ganglion, with a chain of segmental ganglia along its length.
- Sense organs: five pairs of eyes and many small sensory papillae on the skin.
Reproductive system
| Part | Male organs | Female organs |
|---|---|---|
| Gonads | 11 pairs of testes (testis sacs), one pair in each segment from 12 to 22 | One pair of ovaries (ovisacs) in segment 11 |
| Ducts | Vasa efferentia → vas deferens on each side → epididymis (stores sperm) → ejaculatory duct | Oviducts join into a common oviduct → vagina |
| Opening | Male genital pore in segment 10, through the penis | Female genital pore in segment 11 |
| Fertilisation and development | A hermaphrodite, but it mates with another leech: cross-fertilisation, which is internal | Eggs are laid in a cocoon made by the clitellum and left in moist soil; direct development, with no larva |
How to draw the digestive system of a leech
- 1Draw a leech outline and a long straight tube down its middle. Label alimentary canal.
- 2At the front, draw a small round mouth, then a short buccal cavity with three small jaws.
- 3Draw a thick, muscular pharynx with small dots around it for the salivary glands. Then draw a short oesophagus.
- 4Draw a long crop of ten chambers, each with a pair of side pouches (crop caeca). Make the last pair long, reaching backwards.
- 5Then draw a small stomach, a straight intestine, a short rectum and the anus.
Hirudin, the leech's anticoagulant, led to medicines that prevent dangerous blood clots. Leech therapy (jalaukavacharana) is an old Ayurvedic treatment that is still used in some hospitals for swelling and poor blood flow.
2 marks: “How does a leech respire?” Through its moist skin (cutaneous respiration): oxygen diffuses into the haemocoelomic fluid and CO₂ diffuses out.
4 marks: “Describe the reproductive system of a leech.” Hermaphrodite; 11 pairs of testes (segments 12 to 22), ducts to the penis in segment 10; one pair of ovaries in segment 11, oviducts, vagina; cross and internal fertilisation; cocoon; direct development.
7 marks: “Describe the digestive system of a leech with a labelled diagram.” Labelled diagram; mouth in anterior sucker, buccal cavity with three jaws, pharynx with salivary glands (hirudin), oesophagus, crop with chambers and caeca, stomach, intestine, rectum, anus (dorsal, segment 26).
Leech numbers: 17 pairs of nephridia, 11 pairs of testes, 1 pair of ovaries, 3 jaws, 5 pairs of eyes, 33 segments.