Phylum Annelida: important evolutionary changes. Lumbricus terrestris. Hirudo medicinalis

At a glance

1. Annelids are segmented worms: the body is a row of similar segments (metamerism), each with its own muscles, nephridia, ganglia and, in earthworms, chaetae. They have a true coelom, lined all round by mesoderm, and a closed blood system.
2. The fluid of the coelom is a hydrostatic skeleton: waves of contraction of the circular and the longitudinal muscle move the earthworm, its chaetae gripping the soil.
3. Lumbricus terrestris has a prostomium over its mouth, a clitellum that makes the cocoon, a gut with a crop, a gizzard and a typhlosole, five pairs of hearts, a pair of nephridia in most segments and a ventral nerve cord. It is a hermaphrodite that cross-fertilises.
4. Hirudo medicinalis has 32 segments under many rings, a sucker at each end, three toothed jaws and no chaetae; its saliva holds hirudin, which blocks thrombin.
5. Leeches relieve venous congestion in replanted fingers and flaps, with an antibiotic against Aeromonas. Leeches swallowed in untreated water can cling in the nose or throat and bleed.

01The annelids: what changed

The Annelida are over 22,000 species of segmented worms, in the sea, in fresh water and in damp soil. Compared with the flatworms and the roundworms, they bring several new features.

What each brought:

• A true coelom. A fluid-filled cavity between the body wall and the gut, lined on both sides by mesoderm, which also hangs the organs in place. The gut and the body wall can move independently, and the fluid carries food and wastes.
• Metamerism. The body is built of repeated segments, separated inside by walls, the septa, each with its own share of muscles, coelom, nephridia, nerves and vessels. A segment can change shape on its own, and losing one is not fatal.
• A hydrostatic skeleton. The fluid in each segment's coelom cannot be squeezed, so the muscles of the body wall work against it as they would against a bone.
• A closed circulation, with blood moved by vessels that contract, and an excretory organ, the nephridium, in each segment.
• Protostome development. The coelom forms as a split in the mesoderm, a schizocoel. New segments are added one at a time from a growth zone just in front of the tail piece, so the youngest segment is at the back.

The classes

Textbooks divide the annelids into three groups, though studies of their relationships now place the leeches within the oligochaetes and both within the polychaetes:

• Polychaeta, the bristle worms, almost all marine, such as the ragworms, with many chaetae on paired paddles, the parapodia.
• Oligochaeta, the earthworms and their relatives, with few chaetae and no parapodia.
• Hirudinea, the leeches, with suckers and, except for one small group of fish parasites, no chaetae.

The oligochaetes and the leeches are hermaphrodites with a clitellum, and together are called the Clitellata; they hatch from their cocoons as small adults. Most polychaetes have separate sexes, shed their eggs and sperm into the sea, and develop through a swimming larva, the trochophore, which molluscs share.

02Lumbricus terrestris

Lumbricus terrestris, the common earthworm or lob worm, is the largest native earthworm of much of Europe and the example used in biology classes around the world. It is pinkish to reddish brown, generally 110 to 200 mm long and 7 to 10 mm across, with about 120 to 170 segments. It lives in deep vertical burrows and comes up at night to drag leaves down into them.

The outside

• The prostomium, a small fleshy lobe, overhangs the mouth on the first segment, the peristomium. It feels and tastes the way ahead and is not counted as a segment. The anus opens on the last segment.
• Each segment but the first and the last carries eight chaetae, short bristles of chitin, in four pairs: a ventral and a lateral pair on each side.
• The clitellum, a swollen band of glandular skin over several segments a little way back, forms in adult worms and makes the cocoon.
• Small pores open on the segments: a pair of nephridiopores on most of them, dorsal pores that let out coelomic fluid to keep the skin moist, and the pores of the reproductive organs near the front.

Body wall and movement

The body wall, seen in the cross-section of the figure, is a thin cuticle of collagen over an epidermis of one cell layer, with mucous cells; under it a thin layer of circular muscle and a thicker layer of longitudinal muscle; and inside them the peritoneum, which lines the coelom. The worm crawls by peristalsis: where the circular muscle contracts, segments grow long and thin and push forward; where the longitudinal muscle contracts, they grow short and fat and anchor themselves with their chaetae. The waves pass from front to back.

The organ systems

• Digestion. The gut is a straight tube from mouth to anus: a muscular pharynx that sucks in soil and leaves, an oesophagus with calciferous glands, a thin-walled crop that stores food, a muscular gizzard that grinds it with grit, and a long intestine where it is digested and absorbed. A fold of the intestine's roof, the typhlosole, hangs into its lumen and enlarges its surface. Yellow chloragogen cells over the intestine store food and handle wastes, rather like a liver.
• Circulation. A closed system: the dorsal vessel above the gut contracts and drives blood forward; in segments seven to eleven, five pairs of aortic arches, the hearts, pass it down to the ventral vessel below the gut, which carries it back; a subneural vessel runs under the nerve cord. The blood is red with haemoglobin dissolved in the plasma, not in cells.
• Gas exchange. There are no lungs or gills: oxygen crosses the moist skin into its capillaries, which is why the skin must stay moist.
• Excretion. Most segments have a pair of nephridia, here metanephridia, open to the coelom. Each starts as a ciliated funnel, the nephrostome, open to the coelom of the segment in front, and runs through the septum into a long coiled tubule, wrapped in capillaries, which opens through a pore in the side. They excrete ammonia and urea.
• Nervous system. A pair of cerebral ganglia above the pharynx, joined round it to the ventral nerve cord, a double cord with a pair of fused ganglia in each segment. Sense cells in the skin respond to touch, chemicals and light.

Reproduction

Earthworms are hermaphrodites, but each mates with another. In Lumbricus, as in many earthworms, the sexual organs lie in segments 9 to 15. Two pairs of testes lie in segments 10 and 11, inside sacs, the seminal vesicles, and open by male pores on segment 15; a pair of ovaries lies in segment 13 and opens by female pores on segment 14; and two pairs of spermathecae lie in segments 9 and 10. Two worms lie head to tail, joined by mucus from their clitella, and each passes sperm into the other's spermathecae, sacs that store it. Later, the clitellum secretes a ring of mucus that hardens into a cocoon; as the worm backs out of it, it lays its eggs and the stored sperm of its partner into it. Fertilisation is in the cocoon, which closes at both ends and is left in the soil. Development is direct, with no larva: small worms hatch from the cocoon with all their segments. Lumbricus terrestris takes at least a year to become adult, and breeds in its second year.

Earthworms can regenerate lost segments, at the front or the back, to an extent that differs between species.

Earthworms and their water-living relatives are intermediate hosts of some nematodes of animals, among them the giant kidney worm, Dioctophyme renale, which rarely infects people and destroys a kidney.

03Hirudo medicinalis

The medicinal leech, Hirudo medicinalis, lives in ponds and marshes in Europe. It grows up to 20 cm long when stretched out, green to brown, darker on its back, with orange stripes down it. The leeches long sold as H. medicinalis for medicine are often a separate species, Hirudo verbana. Collected for centuries, the European medicinal leech is now near threatened and protected through most of its range.

What sets a leech apart from an earthworm:

• 32 segments in every species, but each mid-body segment carries several rings, the annuli, so the rings outside do not match the segments inside.
• A sucker at each end. The front sucker surrounds the mouth; the broad one at the tail is for holding on. The leech moves by looping between them, like a caterpillar, or swims by waving its body.
• No chaetae.
• A coelom reduced to narrow channels, its fluid doing the work of blood; most of the space is filled with tissue.
• Five pairs of eyes, small eyespots on the head.
• A clitellum, and hermaphrodite sex with cross-fertilisation; the cocoons are laid in damp ground near the water.

How it feeds

The leech fixes its front sucker to the skin of a mammal and cuts it with three jaws, one above and two at the sides, each with a single row of fine teeth, hundreds in all. The bite hurts little, often no more than a slight sting. Its saliva holds more than a hundred proteins:

• hirudin, which blocks thrombin and stops the blood from clotting;
• inhibitors of platelet aggregation;
• substances that widen the vessels and, it is thought, one that numbs the bite.

It fills the side pouches of its crop with blood and drops off. A large meal can last it months, digested slowly with the help of bacteria in its gut, Aeromonas veronii.

Hirudin and the drugs that came from it

Hirudin, a protein of 65 amino acids, is the most potent natural inhibitor of thrombin known. It binds thrombin directly, both free and in clots, and stops it turning fibrinogen into fibrin. Unlike heparin, it needs no antithrombin. It is now made by recombinant methods (lepirudin, desirudin), and bivalirudin, a shorter synthetic peptide modelled on it, is a direct thrombin inhibitor used as an anticoagulant during coronary angioplasty; it carries no risk of heparin-induced thrombocytopenia.

Leeches in medicine

Leech therapy came back in the 1970s and 1980s with microsurgery. After a finger, an ear, a lip or the tip of the nose is sewn back on, or a flap of tissue is moved, the arteries often bring blood in faster than the small or damaged veins can carry it away, and the congestion that follows can kill the tissue within hours; the skin turns dusky purple. Leeches drain it until new veins grow.

• The benefit comes less from the blood the leech drinks than from the bite, which goes on oozing for hours after it drops off. A patient can lose 5 to 15 mL of blood per leech, per session, so the blood count is watched and transfusion may be needed.
• The patient takes an antibiotic against Aeromonas, such as co-trimoxazole or ciprofloxacin, every day of treatment, because the leech's gut bacteria can infect the wound; without it, up to a fifth of patients get a wound infection. Aeromonas resists penicillin, ampicillin and first-generation cephalosporins, and some strains carried by leeches now resist ciprofloxacin.
• Leeches are not used where the tissue lacks arterial inflow, which they cannot help, nor in patients with bleeding disorders or anaemia.
• Each leech is used once and then killed in alcohol.
• In the United States leeches were cleared as a medical device in 2004.

Leeches as parasites

Leeches that feed on blood are temporary ectoparasites of people as well as animals. On the skin they do little harm, and bleeding after the bite stops in hours. Inside the body they are more dangerous: small leeches swallowed in untreated spring or stream water, or entering while bathing, can fasten inside the nose, the pharynx or the larynx and feed there for weeks. This is internal hirudiniasis, caused in the Middle East and around the Mediterranean mainly by nasal leeches of the genus Limnatis, and in South Asia by Dinobdella ferox. It causes repeated nosebleeds, coughing up or vomiting blood, a feeling of something in the throat, trouble swallowing, anaemia and, rarely, a blocked airway. The leech is removed under direct view. Leeches can also carry bacteria, viruses and protozoa from one blood meal to the next for months, so they may act as vectors.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1Which feature appears for the first time in the annelids, among the worms of this course?

Three germ layersA gut with a mouth and an anusA true coelom, lined all round by mesoderm, in a body of repeated segmentsBilateral symmetry

2An earthworm pushes its head forward through the soil. Which muscles contract in its front segments to do it?

The longitudinal muscles, making the segments short and fatThe circular muscles, making the segments long and thinBoth layers at once, making the segments rigidNeither, it pulls itself along with its chaetae alone

3What does the typhlosole of the earthworm do?

Grinds food with swallowed gritStores food before it is groundEnlarges the surface of the intestine that absorbs foodPumps blood from the dorsal to the ventral vessel

4How does hirudin, from the saliva of the medicinal leech, stop blood from clotting?

It binds calciumIt speeds up antithrombin, as heparin doesIt binds thrombin directly and blocks itIt blocks the synthesis of the clotting factors in the liver

5Medicinal leeches are put on a replanted finger that has turned dusky purple after surgery. What else does the patient need while the leeches are used?

Heparin by infusionAn antibiotic active against Aeromonas, such as co-trimoxazole or ciprofloxacin, every day of the treatmentPenicillin, against the bacteria of the skinNothing, since the leeches are sterile