The Phylum Annelida

The Phylum Annelida

Learning Objectives

After studying this chapter, the reader should be able to do the following:

Key Terms




Swimming leech

Stepping leech



Hirudo medicinalis

Beneficial use of leeches

Leeches and earthworms are annelids. Leeches are not considered to be true helminths but are often described as parasitic worms. As external parasites of humans and domesticated and wild animals, leeches are members of the phylum Annelida and the class Hirudinea.


While leeches are considered ectoparasites, one species is used for medicinal purposes.

Hirudinea (Leeches)

Leeches can be predatory or scavenging. Most often they are parasitic on a wide variety of vertebrates and invertebrates. The ectoparasitic leeches feed on blood from fish, crustaceans, frogs, turtles, mollusks, birds, and land animals such as cattle, horses, assorted primates, and humans. These ectoparasites range in size from tiny species that are 5 mm long to varieties such as Haemopis sanguisuga, the horse leech, which has been reported to be as long as 45 cm when extended and swimming.


Leeches have slender, leaf-shaped bodies that lack bristles. The typical leech has two suckers, a large, adhesive posterior sucker and a smaller anterior sucker. The anterior sucker is actually a pseudosucker and surrounds the mouth. As a member of the phylum Annelida, the leech is segmented and lacks a hard exoskeleton; in its place the leech has a thin, flexible cuticle. Because of this thin cuticle, leeches dry out quickly and must always be closely associated with water. A few leeches are found in saltwater; a few terrestrial (land) leeches are found in moist, damp locations. For the most part, leeches should be considered aquatic animals.


The leech has a segmented body that lacks a hard exoskeleton. Instead, it has a thin, flexible cuticle.

As a result of these different habitats, leeches have developed two widely different locomotory habits, swimming and stepping. Swimming is the method of locomotion used when the leech is in water. The leech’s body becomes flattened dorsoventrally as waves of muscular contraction pass down its length. The result is an undulating motion that propels the leech forward.

Stepping is the method of locomotion used when the leech is on solid ground. While in this mode, the leech moves in an “inchworm-like” manner, using its cranial and caudal suckers as organs of attachment to move along the substrate (surface). The layer of circular muscle just beneath the epidermis contracts, and the leech becomes long and thin. The cranial sucker then attaches to the substrate, the caudal sucker releases, and the longitudinal muscle layer beneath the circular muscle brings the caudal sucker up to the vicinity of the cranial sucker, where it attaches. The overall effect is “stepping.”


When on land, the leech moves using an inchworm-like motion called stepping. It alternates using its cranial and caudal suckers to attach to and move along surfaces.

While the leech is attached to the host with its caudal sucker, it uses the cranial sucker to explore the host’s skin to locate a suitable feeding site and to attach tightly. Three rows of jaws with approximately 100 teeth are found in the cranial sucker. These teeth operate similar to a circular saw, penetrating through the skin to a depth of 1.5 mm. The wound produced by the leech bite is a characteristic Y-shaped skin incision. When the incision is made, the host feels very little pain.

In the past, analgesia was thought to be caused by the release of an anesthetic in the leech saliva, but it is now believed that leech saliva does not contain an anesthetic. The ability of a leech to feed is made easier by the secretion of powerful anticoagulants into the site of attachment. A histamine-like substance is added to the wound to prevent the collapse of adjacent capillaries. As blood passes through the mouth, the anticoagulant hirudin is added to it. Hirudin is a 64-amino acid peptide whose function is similar to antithrombokinase. It has been described as the most powerful anticoagulant known. Active agents in the saliva of various species of leeches include a hyaluronidase, a collagenase, and two fibrinases.


Hirudin, one of the most powerful anticoagulants known, is secreted into the bite wound of the leech to keep the host bleeding as the leech feeds.

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Aug 31, 2016 | Posted by in GENERAL | Comments Off on The Phylum Annelida

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