This class includes about 3,500 species of earthworms and freshwater worms. The members of the class range in length from about 1/32 in. to 10 ft (0.5 mm–3 m), but most are comparable to the polychaetes in size. Oligochaetes occur in a variety of habitats throughout the world. Most are burrowers in the soil, but the class also includes worms that inhabit wells, marshes, and swamps. Other species live under rocks on the seashore, in the leaves of tropical trees and vines, on the surface of glaciers, or on the gills of freshwater crayfish.Oligochaete Anatomy
Like the polychaetes, oligochaetes have bodies divided into segments. However, they lack parapodia and, with a few exceptions, have relatively few and inconspicuous setae. The setae are usually arranged in four bundles on each segment; those of aquatic forms are longer than those of land forms. The setae of an earthworm may be felt as a roughness if one rubs a finger along its side.
Oligochaetes are less varied in their external form than the polychaetes, but are much more numerous. As many as 4,000 oligochaetes have been counted in 1 square meter of lake bottom, and about 9,000 in 1 square meter of meadow soil. In almost all oligochaetes, the head is a simple cone-shaped structure without sensory appendages. Light is detected by photoreceptor cells in the skin, usually concentrated toward the front of the animal.
The mouth, located under the head, leads to a relatively simple, straight digestive tract consisting of a pharynx, an esophagus, and an intestine, terminating in an anal opening. Terrestrial oligochaetes tunnel through the ground, swallowing soil as they go. The digestive tract of such a worm is specially modified for this rough diet. Typically it has a thin-walled storage area, or crop, and a muscular gizzard for grinding the soil to remove the organic matter that is the actual food of the worm. Specialized calciferous glands remove excess calcium, magnesium, strontium, and phosphate and regulate the level of these ions in the blood. Solid wastes are egested and plastered against the burrow wall, or ejected from the mouth of the burrow; the ejected material is called castings. Earthworms, through their burrowing and digestive processes, are largely responsible for the mixing and aeration of the soil. Not all oligochaetes have soil diets; some of the small aquatic worms are active predators on other small invertebrates. Excretion is typically carried out by a pair of tubes in each segment.
The circulatory system is that typical of the annelids and has many contractile vessels, or hearts. Although a few aquatic forms have gills for respiration, most oligochaetes lack such specialized structures and use the capillaries of their body walls for respiratory exchange. Oxygen dissolved in the soil water diffuses through the moist epidermis of the worm. If earthworms are forced to the surface, as when their burrows are filled with rainwater, they suffocate as a result of desiccation.
All oligochaetes are hermaphroditic, and nearly all cross-fertilize by copulation. Male and female reproductive organs are located in separate segments. The copulating pair exchange sperm, which are stored in the body of the recipient worm until its eggs are mature. The worm then secretes a cocoon into which it deposits the eggs and the sperm; fertilization and development of the eggs occur in the cocoon. When the young emerge they are miniatures of the adults. The cocoon is secreted by a glandular region, the clitellum, consisting of several thickened segments. The clitellum of an earthworm is a conspicuous saddle-shaped region near its front end.
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