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Segmentation / Annulation

All leeches have 34 somites, which are the "segments" diagnostic for the phylum Annelida. These somites are further segmented into annuli, which are the divisions you will easily recognize when viewing a leech. Each somite contains a certain number of annuli, and this count can be diagnostic for the group or even species. Annuli can also differ in size relative to each other, which is again diagnostic.

Body Plan

Leeches come in a variety of shapes and sizes, with each family typically having a distinct shape. Piscicolids are often recognizable by the stark contrast in width between their suckers and their main body. Some genera of Piscicolidae can also be recognized by their pulsatile vesicles, which are projections that occur laterally along the body.

Many groups have papillae, which can range from small projections to large bumps on the body; their color, number, and arrangement can be diagnostic. In some Placobdella species, the number of pairs of papillae occurring before the posterior sucker is species-specific.

Some Placobdella species — namely P. montifera and P. nuchalis — have their head clearly distinct from the rest of the body, as in Piscicolids, but with the rest of the body being clearly Glossiphoniid in shape.

Eyes / Eyespots

Leeches possess a variety of eye configurations that are of great taxonomic value. The "main" eyes of a leech are typically arrayed in pairs along the anterior end, and their number and arrangement are among the first features examined during identification. In some groups, such as the genus Helobdella, the shape of the eyes can even be characteristic of specific species.

Supplementary eyes, also called accessory or lateral eyes, occur in some taxa along the lateral or dorsolateral margins of the anterior somites. These are smaller than the main anterior ocelli and their presence, number, and position can be useful for distinguishing families and genera. They are particularly notable in certain Piscicolidae, where rings of lateral eyes may encircle the anterior body somites.

Eyes on the caudal sucker can be found on some Piscicolid leeches. These posterior ocelli occur on or around the posterior sucker and are a distinctive synapomorphy of certain lineages within Piscicolidae.

Anterior Sucker / Oral Sucker

The anterior sucker is a muscular adhesive disc at the front end of the leech, formed from the prostomium and the first few anterior somites, and it encompasses or surrounds the mouth. The size relative to the body, overall shape, and the degree to which it is set off from the body are diagnostic in some groups. In Piscicolidae the anterior sucker is often dramatically narrower than the body and clearly demarcated, while in Glossiphoniidae it grades more gradually into the forebody.

Digitate processes are finger-like projections that extend from the margin or inner surface of the anterior sucker in Actinobdella species. The number of them is diagnostic to species.

Mycetomes

Mycetomes (also called bacteriomes) are specialized organs housing endosymbiotic bacteria or other microorganisms. In leeches, mycetomes have been documented primarily in sanguivorous groups, where they are thought to house bacteria that supplement the leech's diet.

Structurally, mycetomes are typically paired organs lying in the lateral body wall or associated with the digestive tract, often visible as yellowish or whitish bodies in cleared or fresh specimens. In medicinal leeches (Hirudo medicinalis and related species) and in some Haemopidae, mycetomes occur as discrete structures associated with the crop or intestine. Their presence, position, and development can vary among families; they are generally better developed and more easily identified in obligate blood-feeders than in predatory leeches.

Mouth / Jaws / Proboscis

At the highest level, leeches are divided into those with jaws (Gnathobdellida) and those with a proboscis (Rhynchobdellida), with a third group, the Pharyngobdellida (swallowing predators), lacking both well-developed jaws and a muscular eversible proboscis.

Jaws are present in Gnathobdellid leeches as two or three muscular, blade-like structures armed with teeth. In tri-jawed forms such as Hirudo and Macrobdella, one jaw is dorsal and two are ventrolateral, producing the characteristic "Y-shaped" wound. In bi-jawed forms (e.g., Haemopis) the jaws are less well developed or functionally reduced. The jaws are composed of a firm, cartilaginous connective tissue, and their cutting edges bear teeth whose number, size, and spacing are important diagnostic characters at the genus and species level.

Denticles are smaller, secondary tooth-like structures that may occur between the primary teeth or along the jaw margin in certain taxa. Their presence and number can further refine identification, particularly when primary tooth counts overlap between species.

In Rhynchobdellida there is a proboscis instead of jaws. The proboscis is a muscular, eversible tube that extends through the oral pore and is used to penetrate the host's skin. In Piscicolidae it is typically long and slender; in Glossiphoniidae it is shorter and somewhat broader. The proboscis itself lacks teeth, but may bear minute internal ridges. When retracted, it lies in a muscular proboscis sheath (the rhynchocoel) and can be observed in cleared or dissected specimens. The oral pore of rhynchobdellid leeches tends to be a small, central opening rather than the large gape seen in gnathobdellids, and the entire anterior sucker structure reflects this difference in feeding mode.

In pharyngobdellid leeches neither functional jaws nor a proboscis is present. These leeches swallow small invertebrate prey whole or in large pieces using a muscular, non-eversible pharynx. The pharynx is highly muscular and may be ridged internally, and its length and musculature are characters used in higher-level systematics. In some erpobdellid genera, reduced jaw-like structures ("jaw rudiments") are present but aren't functional for cutting.

Salivary Glands

Salivary glands in gnathobdellid leeches are typically paired or unpaired clusters of secretory cells associated with the pharyngeal region and extending posteriorly in the body. In Hirudinidae and Haemopidae they form extensive glandular masses visible on dissection. In rhynchobdellid leeches, analogous pharyngeal glands are present. The number, arrangement, and extent of salivary and pharyngeal glands vary among families and may be used in higher-level identification.

Clitellum

The clitellum is a glandular region of the body wall present in all clitellate annelids. It is composed of specialized epidermal cells that produce the cocoon material (albumin and mucus) that surrounds the eggs during reproduction. In leeches, the clitellum is typically functional only during the breeding season, during which it becomes distinctly thickened and glandular; outside of this period it may be difficult to recognize.

Gonopores, Accessory Pores & Nephridiopores

Gonopores are the external openings of the reproductive system. Leeches have both male and female gonopores, situated ventrally on the midline. The male gonopore opens anteriorly, typically on somite XI, followed by the female gonopore, which opens on somite XII in most species, though the exact somites vary by group. The distance between the male and female gonopores is a commonly used and often diagnostically important character. This distance is measured in annuli (subsegments) and can range from quite large to zero, where the gonopores are united.

Leeches can also have accessory pores beyond the normal two gonopores. These can present as extra singular pores anterior and posterior to the normal pair (as in Barbronia species) or as a grouped region of multiple pores, often on raised beds (as in Macrobdella).

Nephridiopores are the external openings of the excretory organs, arranged segmentally along the ventral or ventrolateral body surface. Each somite typically bears one pair of nephridiopores, though in some groups or segments they may be absent, fused, or reduced. Nephridiopores often appear as tiny pores along a regular lateral or ventrolateral line and are useful for segment counting.

Crop

The crop is a large, expandable storage organ of the midgut that provides more storage and surface area for digestion. Food stored in the crop can be digested over weeks or months, allowing leeches with large crops to go long times without eating. The crop can range from large with many branches to a simple transport tube.

Crop diverticula are lateral pouches that branch off the sides of the main crop lumen, dramatically increasing its storage capacity. The precise number of diverticulum pairs and whether they are simple or branched can be diagnostic to family, genus, and sometimes species.

Crop caeca are lateral pouches at the posterior end of the crop or intestine, distinct from the lateral diverticula. Their presence, number and relative length are diagnostic in many groups. In some Helobdella, the shape of the outline of the caeca (jagged vs smooth) can be species specific.

Atrium

The atrium is the common chamber into which the male reproductive ducts open, serving as the final portion of the male reproductive tract before the gonopore. It is typically a muscular, thick-walled sac lying just internal to the male gonopore. In gnathobdellid leeches, the atrium is associated with a penis bulb or penis sheath, and the muscular atrium forces the penis outward during copulation.

In rhynchobdellid leeches, which lack a penis, the atrium is modified accordingly and mainly channels sperm toward spermatophore-producing glands. In Erpobdellidae the atrium is also well developed and is associated with a penis that may be very long and coiled. The length of the uncoiled penis and the number of coils are frequently used in erpobdellid taxonomy.

Atrial cornua are paired, horn-like or tubular projections extending from the atrium. In Glossiphoniids, the cornua project laterally or posterolaterally from the atrium and are thought to function in puncturing the partner's body wall during hypodermic insemination, since Glossiphoniid leeches lack a penis and transfer sperm via spermatophores or direct injection.

Female Reproductive System

The female reproductive system of leeches is considerably simpler in external structure than the male system but is nonetheless important in taxonomy and systematics.

Oviducts are the paired ducts leading from each ovisac toward the midline, where they unite into a common oviduct that opens at the female gonopore. In some taxa an expanded region of the oviduct, sometimes called the uterus or vagina, is present just proximal to the gonopore and may be involved in egg storage, sperm reception, or cocoon formation. The relative length and any specialization of the oviduct are useful when present.

Ovisacs are paired sac-like structures that enclose the ovaries and developing oocytes. They are connected to the common oviduct and the female gonopore. Ovisac number, shape, and size relative to the body are occasionally useful characters.

Male Reproductive System

The male reproductive system of leeches is typically complex and, together with the number and position of testisacs, provides some of the most reliable internal characters for identification.

Testisacs (also called testes or testicles) are paired, sac-like structures that produce sperm. In most leeches they are arranged in a single linear series of pairs along the body, and the number of testisac pairs is one of the most important diagnostic characters. The number ranges from as few as four pairs (in some Piscicolidae) to ten or more pairs (in Hirudinidae).

Efferent ducts lead from each testisac to a vas deferens (also called sperm duct) on each side of the body, and the vas deferens carries sperm anteriorly toward the atrium. The epididymis is a coiled portion of the vas deferens present in some taxa, that is thought to be associated with sperm maturation and storage. A prominent, elongated epididymis is particularly characteristic of certain Hirudinidae and Haemopidae and, when present, its size and degree of coiling are diagnostic.

The pre-atrial loop is a distinctive coil or loop of the vas deferens or ejaculatory duct immediately proximal to the atrium, found in certain groups. It is most notable in some Erpobdellidae, where the pre-atrial loop (combined with penis length and coiling) is an important character for diagnosis.

The ejaculatory duct is the final, typically narrower section of the male reproductive duct, running from the vas deferens into the atrium. It may be glandular, muscular, or both. In some taxa it opens directly into the atrial lumen; in others it connects to the base of the penis.

Posterior Sucker

The posterior sucker is a muscular adhesive disc at the caudal end of the leech, formed from fused posterior somites (typically somites XXVI–XXXIV). It functions primarily in locomotion and temporary attachment to hosts or substrates. The posterior sucker is generally larger relative to the body than the anterior sucker.

The anus is the terminal opening of the digestive tract, located dorsally on the last true body somite (somite XXXIV or so), just anterior to the base of the posterior sucker. The anus is typically a small, inconspicuous pore; its position is primarily of use as an anatomical landmark confirming the boundary between the body proper and the fused posterior sucker segments, and in confirming the presence or absence of the posterior gut and rectum in dissected leeches.

The diameter of the posterior sucker relative to body width is a useful character: in Piscicolidae it may be dramatically wider than the slender body, while in Erpobdellidae it is typically more modestly sized. The posterior surface of the sucker may bear concentric ridges or papillae that assist adhesion, or even have eyespots as in some Piscicolids.

The posterior sucker can be attached directly to the main body, or separated by a distinct pedicel, which has evolved separately many times and can be diagnostic, such as its presence in Placobdella pediculata.

Specialized Structures

Scutes are hardened, plate-like structures found notably in the nuchal region of some Helobdella species. This plate can vary in color, size, and shape, and is useful for separating Helobdella species where the information on it exists.

A highly specialized feature found in Gastrostomobdellids is the presence of a secondary mouth, dorsally positioned, distinct from the ventral primary mouth that is present in most other leeches. This gastrostome opens between the dorsal lips of the anterior body and connects to the pharynx. This structure appears to be an adaptation for feeding in these largely terrestrial or semi-terrestrial Asian leeches.