An Introduction to Class Oligochaeta

Class Oligochaeta

More than 3000 species of oligochaetes are found in a great variety of sizes and habitats. They include the familiar earthworms and many species that live in freshwater. Most are terrestrial or freshwater forms, but some are parasitic, and a few live in marine or brackish water. With few exceptions, oligochaetes bear setae, which may be long or short, straight or curved, blunt or needlelike, or arranged singly or in bundles. Whatever the type, setae are less numerous in oligochaetes than in polychaetes, as is implied by the class name, which means “few long hairs.” Aquatic forms usually have longer setae than do earthworms.

Form and Function The main features of an oligochaete body are described with reference to the familiar earthworm. The circulatory system and excretory structures described in earthworms are typical of annelids in general, but the digestive and nervous systems have aspects specific c to oligochaetes.

Earthworms, sometimes called “night crawlers,” burrow in moist rich soil, and usually live in branched, interconnected tunnels. The species commonly studied in laboratories is Lumbricus terrestris (L. lubricum, earthworm). It ranges in size from 12 to 30 cm long, but is small in comparison to giant tropical forms whose 4-m-long bodies may comprise 150 to upward of 250 segments.

Earthworms normally emerge at night, but in damp rainy weather they stay near the surface, often with mouth or anus protruding from the burrow. In very dry weather they may burrow several feet underground, coil in a slime chamber and become dormant.

Earthworms use peristaltic movement: Contractions of circular muscles in the anterior end lengthen the body, pushing the anterior end forward where it anchors. Anchoring is accomplished by contraction of the longitudinal muscles in forwarsegments—these segments become short and wide, pushing against the sides of the burrow. As they do so, bristlelike rods called setae project outward through small pores in the cuticle. Setae dig into the walls of the burrow to anchor the forward segments; contractions of longitudinal muscles then shorten the rest of the body, pulling the posterior end up behind the anchored anterior region. As waves of extension and contraction pass along the entire body, it gradually moves forward.

The paired epidermal setae of oligochaetes are set in a sac within the body wall and moved by muscles, as they are in polychaetes. However, oligochaetes do not have parapodia; instead the setae extend directly out of the body wall on each segment. In most earthworms each segment bears four pairs of chitinous setae, although there may be more than 100 such setae per segment in some oligochaetes.

Aristotle called earthworms the “intestines of the soil.” Some 22 centuries later Charles Darwin published his observations in his classic The Formation of Vegetable Mould Through the Action of Worms. He showed how worms enrich soil by bringing subsoil to the surface and mixing it with topsoil. An earthworm can ingest its own weight in soil every 24 hours, and Darwin estimated that from 10 to 18 tons of dry earth per acre pass through their intestine annually, thus bringing potassium and phosphorus from the subsoil and also adding nitrogenous products to the soil from their own metabolism.

They also drag leaves, twigs, and organic substances into their burrows closer to the roots of plants. Their activities are vitally important in aerating soil. Darwin’s views were at odds with his contemporaries, who thought earthworms were harmful to plants. But recent research has amply confirmed Darwin’s findings, and earthworm management is now practiced in many countries.

Nutrition

Most oligochaetes are scavengers. Earthworms feed mainly on decaying organic matter, bits of leaves and vegetation, refuse, and animal matter. After being moistened by secretions from the mouth, food is drawn inward by the sucking action of their muscular pharynx. The liplike prostomium aids in manipulating food into position. Calcium from soil swallowed with food tends to produce a high blood calcium level. Calciferous glands along the esophagus secrete calcium ions into the gut and so reduce the calcium ion concentration of their blood. Calciferous glands also function in regulating acid-base balance of body fluids.

Leaving the esophagus, food is stored temporarily in the thin-walled crop before being passed on to the gizzard, which grinds food into small pieces. Digestion and absorption occur in the intestine. The wall of the intestine is infolded dorsally to form a typhlosole, which greatly increases the absorptive and digestive surface. Surrounding the intestine and dorsal vessel and filling much of the typhlosole is a layer of yellowish chloragogen tissue (Gr. chlo¯ros, green, + ago¯ge¯, a carrying away).

This tissue serve as a center for synthesis of glycogen and fat, a function roughly equivalent to that of liver cells. When full of fat, chloragogen cells are released into the coelom where they float freely as cells called eleocytes (Gr. elaio, oil, + kytos, hollow vessel [cell]), which transport materials to the body tissues. Eleocytes can pass from segment to segment and may accumulate around wounds and regenerating areas, where they break down and release their contents into the coelom. Chloragogen cells also function in excretion.

Circulation and Respiration Annelids have a double transport system: coelomic fluid and a closed circulatory system. Food, wastes, and respiratory gases are carried by both coelomic fluid and blood in varying degrees. Blood circulates in a closed system of vessels, which includes capillary systems in the tissues. Five main blood trunks run lengthwise through the body.

A single dorsal vessel runs above the alimentary canal from the pharynx to the anus. It is a pumping organ, provided with valves, and it functions as a true heart. This vessel receives blood from vessels of the body wall and digestive tract and pumps it anteriorly into five pairs of aortic arches. The function of aortic arches is to maintain a steady pressure of blood in the ventral vessel.

A single ventral vessel serves as an aorta. It receives blood from the aortic arches and delivers it to the brain and rest of the body, providing segmental vessels to the walls, nephridia, and digestive tract. Their blood contains colorless ameboid cells and a dissolved respiratory pigment, hemoglobin. The blood of some annelids may have respiratory pigments other than hemoglobin, as noted. Earthworms have no special respiratory organs, but gaseous exchange occurs across their moist skin.

Excretion

Each segment (except the first three and the last one) bears a pair of metanephridia. Each metanephridium occupies parts of two successive segments. A ciliated funnel, the nephrostome, lies just anterior to an intersegmental septum and leads by a small ciliated tubule through the septum into the segment behind, where it connects with the main part of the nephridium.

Several complex loops of increasing size compose the nephridial duct, which terminates in a bladderlike structure leading to an opening, the nephridiopore.  The nephridiopore opens to the outside near the ventral row of setae. By means of cilia, wastes from the coelom are drawn into the nephrostome and tubule, where they are joined by salts and organic wastes transported from blood capillaries in the glandular part of the nephridium. Waste is discharged to the outside through a nephridiopore.

Aquatic oligochaetes excrete ammonia; terrestrial oligochaetes usually excrete the much less toxic urea. Lumbricus produces both, the level of urea depending somewhat on environmental conditions. Both urea and ammonia are produced by chloragogen cells, which may break off and enter the metanephridia directly, or their products may be carried by the blood. Some nitrogenous waste is eliminated through the body surface.

Oligochaetes are largely freshwater animals, and even such terrestrial forms as earthworms must exist in a moist environment. Osmoregulation is a function of the body surface and the nephridia, as well as the gut and dorsal pores. Lumbricus will gain weight when placed in tap water and lose it when returned to soil. Salts as well as water can pass across the integument, salts apparently being actively transported.

Nervous System and Sense Organs

The nervous system in earthworms consists of a central system and peripheral nerves. The central system reflects the typical annelid pattern: a pair of cerebral ganglia (the “brain”) above the pharynx, a pair of connectives passing around the pharynx connecting the brain with the first pair of ganglia in the nerve cord; a solid ventral nerve cord, really double, running along the floor of the coelom to the last segment; and a pair of fused ganglia on the nerve cord in each segment. Each pair of fused ganglia provides nerves to the body structures, which contain both sensory and motor fibers.

Neurosecretory cells have been found in the brain and ganglia of both oligochaetes and polychaetes. They are endocrine in function and secrete neurohormones concerned with the regulation of reproduction, secondary sex characteristics, and regeneration. For rapid escape movements most annelids have from one to several very large axons commonly called giant axons, or giant fibers, located in the ventral nerve cord. Their large diameter increases rate of conduction and makes possible simultaneous contractions of muscles in many segments.

In the dorsal median giant fiber of Lumbricus, which is 90 to 160 m in diameter, speed of conduction has been estimated at 20 to 45 m/second, several times faster than in ordinary neurons of this species. This is also much faster than in polychaete giant fibers, probably because in earthworms the giant fibers are enclosed in myelinated sheaths, which insulate them.

Simple sense organs are distributed all over the body. Earth worms have no eyes but do have many lens-shaped photo receptors in their epidermis. Most oligochaetes are negatively phototactic to strong light but positively phototactic to weak light. Many single-celled sense organs are widely distributed in the epidermis. What are presumably chemoreceptors are most numerous on the prostomium. In the integument are many free nerve endings which are probably tactile in nature.

General Behavior Earthworms are among the most defenseless of creatures, yet their abundance and wide distribution indicate their ability to thrive. Although they have no specialized sense organs, they are sensitive to many stimuli. They react positively to mechanical stimuli when such stimuli are moderate and negatively to a strong stimulus (such as footfall near them), which causes them to retire quickly into their burrows. They react to light, which they avoid unless it is very weak. Chemical responses aid them in the choice of food. Chemical as well as tactile responses are very important to earthworms. They not only must sample the organic content of soil to find food, but also must sense its texture, acidity, and calcium content.

Experiments show that earthworms have some learning ability. They can be taught to avoid an electric shock, and thus can develop an association reflex. Darwin credited earthworms with a great deal of intelligence because they pulled leaves into their burrows by the narrow end, the easiest way for drawing a leaf-shaped object into a small hole. Darwin assumed that seizure of leaves by worms did not result from random handling or from chance but was deliberate. However, investigations since Darwin’s time have shown that the process is mainly one of trial and error, for earthworms often seize a leaf several times before getting it right.

Reproduction and Development

Earthworms are monoecious (hermaphroditic); both male and female organs are found in the same animal. In Lumbricus reproductive systems are found in segments 9 to 15. Two pairs of small testes and two pairs of sperm funnels are surrounded by three pairs of large seminal vesicles. Immature sperm from the testes mature in seminal vesicles, then pass into sperm funnels and down sperm ducts to the male genital pores in segment 15, where they are expelled during copulation. Eggs are discharged by a pair of small ovaries into the coelomic cavity, where ciliated funnels of the oviducts carry them outside through female genital pores on segment 14. Two pairs of seminal receptacles in segments 9 and 10 receive and store sperm from the mate during copulation.

Reproduction in earthworms may occur throughout the year as long as warm, moist weather prevails at night. When mating, worms extend their anterior ends from their burrows and bring their ventral surfaces together. Their surfaces are held together by mucus secreted by the clitellum (L. clitellae, packsaddle) and by special ventral setae, which penetrate each other’s bodies in the regions of contact. After discharge, sperm travel to seminal receptacles of the other worm via its seminal grooves. After copulation each worm secretes first a mucous tube and then a tough, chitinlike band that forms a cocoon around its clitellum.

As the cocoon passes forward, eggs from the oviducts, albumin from skin glands, and sperm from the mate (stored in the seminal receptacles) pour into Fertilization of eggs then occurs within the cocoon. When the cocoon slips past the anterior end of the worm, its ends close, producing a sealed, lemon-shaped body. Embryogenesis occurs within the cocoon, and the form that hatches from the egg is a young worm similar to the adult. Thus development is direct with no metamorphosis. Juveniles do not develop a clitellum until they are sexually mature.

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Useful External Links

• Life forms in Oligochaeta: A literature review by Timm Tarmo
• Global diversity of oligochaetous clitellates (“Oligochaeta”; Clitellata) in freshwater by Adrian Pinder
• Ecology and Biology of Marine Oligochaeta – an Inventory Rather than another Review by Olav Giere