Medicinal Leech Anatomy
External and internal anatomy — 32 segments, tripartite jaw, and crop-based blood storage
Last Updated: March 5, 2026Reviewed by: Andrei Dokukin, MD
Last updated: March 14, 2026
The medicinal leech (Hirudo medicinalis) is a highly specialized hematophagous annelid with anatomical adaptations for blood feeding and storage. Understanding these structures is essential for clinical use and infection control.
External Anatomy
- 32 segments — each segment subdivided into 5 annuli (total 160 body rings)
- Anterior sucker — oral sucker containing three muscular jaws arranged at 120° angles
- Posterior sucker — caudal sucker used for attachment and locomotion (no jaws)
- 5 pairs of eyes — arranged in an arc on the dorsal surface of the anterior sucker; photosensitive organs used for detecting shadows (predator avoidance)
- Dorsal pores — nephridiopores and dorsal pores for excretion
- Gonopores — male and female reproductive openings (hermaphroditic)
The segmented body plan allows remarkable flexibility — leeches can contract to 1/3 of extended length or stretch to 3x resting length.
Internal Anatomy
- Crop (gastric cecum) — stores 5-10x body weight in blood; largest internal organ. Blood remains liquid for months through anticoagulant enzymes.
- 10 pairs of intestinal diverticula — extend from crop, increase storage capacity
- Coelomic sinuses — no true circulatory system; fluid-filled cavities transport nutrients and oxygen
- 17 pairs of nephridia — excretory organs analogous to kidneys
- Hermaphroditic reproductive system — both ovaries and testes present; reproduction via cocoon deposition
- Ventral nerve cord — 21 segmental ganglia connected by paired nerve cords
The Jaw Apparatus
The tripartite jaw system is the defining anatomical feature of Hirudo and other jawed leeches (Arhynchobdellida):
- 3 muscular jaws arranged at 120° angles (dorsal + 2 ventrolateral)
- ~80 chitinous teeth per jaw (total ~240 teeth)
- Y-shaped incision — creates characteristic triradiate wound (often called "Mercedes-Benz" pattern)
- Salivary ducts — open between teeth, delivering anticoagulant cocktail directly into wound
The jaw musculature can generate significant force — up to 10 Newton (~1 kg force) — allowing penetration through epidermis into capillary beds.
Wound Geometry Optimization
The triradiate jaw creates a wound geometry optimized for both active blood extraction (capillary rupture) and prolonged passive bleeding (large surface area for anticoagulant diffusion).
Anatomical Summary Table
| Organ/Structure | Location | Function | Clinical Relevance |
|---|---|---|---|
| Crop | Mid-body (segments 10-18) | Blood storage (5-10x body wt) | Sustains leech for months between feedings |
| Tripartite jaws | Anterior sucker | Y-shaped incision | Creates optimal wound for blood flow + anticoagulant delivery |
| Salivary glands | Paired, along jaws | Secrete 434+ bioactive proteins | Anticoagulation, vasodilation, anti-inflammatory effects |
| Ventral nerve cord | Ventral midline | Central nervous system (21 ganglia) | Sensory processing, host detection |
| Nephridia (17 pairs) | Segmentally distributed | Excretion of metabolic waste | Maintains homeostasis during prolonged digestion |
| Eyes (5 pairs) | Dorsal anterior | Photoreception (shadow detection) | Predator avoidance; negative phototaxis aids attachment |
Educational Disclaimer
This page describes biological properties of medicinal leeches for educational purposes. Discussion of biological mechanisms does not constitute evidence of therapeutic efficacy.