The phylum Annelida includes three main groups: the earthworms, the leeches, and the bristleworms. Annelids are found worldwide, and inhabit terrestrial, freshwater, and marine ecosystems . There are over 15,000 described species.
Characteristics of Annelids
All annelids are segmented. Segments, also called metameres, are structures that occur repeatedly along the body of the animal. Each annelid segment contains units of the circulatory, nervous, and excretory systems. In the earthworms and bristleworms, but not the leeches, segmentation extends to the interior of the body, and includes the coelom , which is partially divided into units by structures called septa.
The annelid body is covered by a moist outer cuticle that is secreted by the epidermis . Both earthworms and bristleworms also possess hairlike setae, composed of chitin , the hard material that also forms the exoskeletons of insects. These are absent in leeches.
Annelids have a true coelom, that is, one that is lined with cells originating from the embryonic mesoderm . The coelom is fluid-filled, which creates hydrostatic (water) pressure and acts as a hydrostatic skeleton . Annelids have a well-developed, closed circulatory system (one in which blood is limited to vessels) that is segmentally arranged. They also have a complete, one-way digestive tract with a mouth and anus. The digestive tract is not segmented.
Respiration in annelids occurs primarily through their moist skin, although certain species have evolved specialized gills or use paired projections called parapodia in gas exchange. The annelid excretory system consists of paired nephridia found in each segment which function in excreting nitrogenous waste. In terms of nervous system structure, annelids possess a pair of ganglia (masses of nerve tissue) at the front end of the body; this serves as their brain. A double nerve cord runs along the ventral (belly) side of the body, and sends branches into each segment. Annelids have many types of sensory receptors, including tactile (touch) receptors, chemoreceptors (smell or taste), and photoreceptors for light. Some have well-developed eyes.
Annelids possess both circular and lengthwise muscle fibers. These, combined with their segmentation and hydrostatic skeleton, allow for great flexibility in movement. One part of the body is able to contract, or change its diameter and length, without affecting the rest of the body. It is believed that the need for elaborate mechanisms to control motion led to the development of the comparatively complex nervous system of annelids.
Some annelids are hermaphroditic while others are dioecious , that is, the sexes are separate. Some species have direct development, in which eggs develop directly into miniature versions of the adult. In other species, there is a larval stage. The annelid larval form is called the trochophore larva. Some annelid species can also reproduce asexually by budding .
Classes of Annelids
Annelids have been divided into three classes. The Polychaeta is exclusively composed of the bristleworms, the Oligochaeta the earthworms, and the Hirudinea include the leeches.
The Polychaeta, or bristleworms, are a large and diverse group that includes polychaete worms, lugworms, ragworms, and sandworms, among other groups. It is the largest annelid class, with over 10,000 species, most of which are marine. Bristleworms are found in a wide variety of habitats and employ various feeding strategies. There are active burrowers whose habitat is at the bottom of the water, that which live within tubes they secrete, and pelagic (open ocean-dwelling) forms. Some are sedentary filter feeders that extract small food particles from the water while others process sediment. Also, some species are active predators; these generally prey on small invertebrates.
Bristleworms are characterized by paired paddle-like appendages called parapodia, used for gas exchange. These are covered with setae ("polychaete" means "many hairs"). Bristleworms have a well-developed head region, often with tentacles, and well-developed sense organs, including paired eyes, antennae, and sensory palps (projections). They are unusual among annelids because their reproductive organs are developed only during the breeding season; afterward, they wither away. The sexes are separate. Gametes (eggs and sperm) are shed into the water, and fertilization is external. Development is indirect, via a trochophore larval stage.
The polychaetes are believed to be the most primitive of the annelid classes. Some species, however, are highly specialized.
The class Oligochaeta includes the familiar terrestrial earthworms, found just about everywhere, as well as some freshwater annelid species. Approximately 3,000 oligochaete species have been described.
"Oligochaete" means "few hairs," and oligochaete species generally have fewer setae than bristleworms. Oligochaetes lack parapodia, eyes, and tentacles. Many aquatic oligochaete species have gills to aid in gas exchange. Species typically feed on debris and algae. Earthworms are critical components of land-based ecosystems. By passing large quantities of soil through their guts, they speed the rate of nutrient turnover. Their burrowing activity also supplies the soil with air.
Most oligochaete species are hermaphroditic, with each individual producing both eggs and sperm. Earthworms, however, generally do not self-fertilize. During mating, two worms line up next to each other, with swollen regions called clitella placed next to each other. Sperm is released through grooves in the skin by both individuals, and these are passed to sperm receptacles in the other worm. The clitellum of each then secretes a ring of mucus that carries eggs from the oviduct (a tube for transporting eggs) and collects sperm from the sperm receptacles. This ring slides over the head of the worm, drops into the soil, and closes off, forming a cocoon. Fertilization takes place within the cocoon and a few eggs hatch two weeks later. Development in earthworms, as well as in the other oligochaetes, is direct, without a larval stage.
The Class Hirudinea consists of the leeches. Leeches differ from other annelids in that most have a fixed number of segments. Leeches lack the hairlike setae of the other annelids and their bodies are somewhat dorsoventrally flattened (i.e., in such a way that the back and belly are close together). As with the oligochaetes, leeches are primarily hermaphroditic and exhibit direct development. There are about 500 described species.
Most leeches are aquatic, and of these, nearly all are found in freshwater environments. A few species are terrestrial, but are found only in fairly warm, moist habitats. Leeches are almost all ectoparasites , which attach to the external surface of the host (as opposed to endoparasites , which live within their hosts). Segments at the front end of the animal are specialized to form suckers, while back-end segments are specialized for attaching to the host. The mouth contains teeth that are used to make an incision in the host. Leeches secrete an anticoagulant that keeps the blood of their host from coagulating, or clotting. They have been put to medical uses for thousands of years. In fact, bloodletting was extremely common as a standard prescription for a wide variety of ailments. The anticoagulants produced by leeches are still of great interest to medical scientists.
see also Phylogenetic Relationships of Major Groups.
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Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.
Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.