By Muhammad Adil Khan

Diagnostic Characteristics that define Annelida
Organism of the phyla Annelida, which means “little rings” are characterized by a segmented body. There are about 15,000 species of Annelida that range from 1mm to 3-m in length. These types of organism are commonly found in most freshwater biomes and damp soil. Annelids also make up the majority of macrofauna (animals large enough to be seen with the naked eye) in the marine environment. (RF 22) Their bodies consist of a head on the anterior end of the body and many segments. Each segment holds a fluid filled area that contains a digestive tract and a tissue layer lining coelom that suspends internal organs. Each segment is separated by septum. Annelids typically have a body wall made up of a variety of different types of muscles, including circular and longitudinal muscle fibers. This body wall, while segmented, is covered by a moist cuticle which is released by an epidermal epithelium. (16)(MG) Annelids are segmented both externally and internally as well and many of the internal structures are repeated in each segment. Each segment has four pairs of setae, which are mini hair like structures around the body that are used for locomotion. The setae are necessary for they provide traction necessary for the earthworm to move. Without this traction, the slick earthworm body would not have much control over its directional travels. The setae are small enough, however, to not detract from the annelid’s key ability to burrow; Setae also can be extended of contracted. (JR) 28 The general anatomy of Annelids can be shown below in Figure 1 with the most common member of the phylum: the earthworm. (1)
Anatomy of the Earthworm (1)
Anatomy of the Earthworm (1)
Anatomy of Annelids:
Annelids have bilateral symmetry. Their bodies can be divided into three regions: head, trunk and terminal. The external wall of annelids in the trunk region is divided into a series of linear cylindrical segments. This division pattern is also followed in the interior of the body. Therefore, every segment on the outer region of the body also corresponds to the internal division. (MStern) 23
Separating each segment in many annelids is a septum, a wall separating each segment. The species with well formed septa, circulate blood entirely within blood vessels. Often times, vessels in the front and rear of the annelid will act as hearts. (JCH 23)
However, the guts of annelids are unsegmented and run from the mouth (located underside the head) through the middle of the body to the anus (on the pygidium). The gut is separated from the body wall by the coelom. Septa, which separate the segmented compartments of the coelom, are perforated by the gut and blood vessels. With the exception of leeches, the coelom is filled with fluid and functions as a skeleton. This provides rigidity and resistance necessary for muscular movement. If the worm is punctured, it cannot move properly as body muscles need the fluid volume in the coelom to function. (VN 25)

Although there are some small segmented worms belonging to the phylum Annelida that lack a seperated cirrculatory system, most have a well developed, closed system. This works through the use of two contracting vessels, dorsal and ventral, which move the annelid's blood around the veins surrounding its gut. There are multiple verticle segmented vessels, often reffered to as "hearts", which help pump blood to the ventral vessel (posterior). The system segments to reach respitory surfaces and the excretory system. (BF 24)

The Body Wall of Annelids:
(18) The body wall of the annelids are covered by an epidermis layer that is overlaid by a thin pliant cuticle that is secreted by the epidermal cells. The body wall is consisted of well developed and segmentally arranged muscles that are used for locomotion (crawling and swimming). Most of the creatures, in the phylum Annelida, contain short external bristles called setae, or chaetae, that are both composed of chitin. (NM 18)

Annelids are divided into three classes:
Now that we’ve established the characteristics of Annelids, we will now dive into the three classes of the Annelida phylum; Oligochaeta, Polychaeta, and Hirudinea.

Class Oligochaeta
This class includes the earthworms and a variety of aquatic species. The main characteristics that define this class are that these terrestrial and aquatic worms have a reduced head, no parapodia, which are un-jointed, lateral outgrowths from the bodies and setae, which are used for locomotion. (1) Oligochaetas are defined by their segmentation. The segments are divided by membrane partitions. Except for the first and last segment, the segments mostly look alike. The first segment, called the Prostomium, contains most of the sensory organs of the animals. The last is called the Pygidium, and contains the anus. It is also worthy to note that the Pygidium is not a true segment, as it does not contain the same sets of organs, nor does it develop in the same way as other segments. (GR)(17) Earthworms are the most common Oligochaetas. (AJM)(13)

An Earthworm, a common Annelid (
An Earthworm, a common Annelid (

Class Polychaeta
Polychaetes are otherwise known as marine worms, and various species include lugworms, clam worms, bristleworms, fire worms, and sea mice. (AS 21) Polychaetes contain segments that have a pair of paddle-like structures called parapodia that function in locomotion. Each parapodium contains setae made of polysaccharide chitin. In many Polychaeta, the parapodia act as gills. The majority of Polychaetes are aquatic organism, with plankton and some in bits of sand or broken shells. Other major characterizes of this class include well-developed heads, and most are tube-dwelling and free-living. (1)

Class polychaeta is comprised of about 80 families and the families are often assigned into one of two artificial groups of Errentia and Sedentaria. Errantia include worms with an errant, or freely moving, lifestyle. The Sedentaria are mostly tube-dwelling or burrowing polychaetes who possess reduced parapodia with some of their setae, which are bristle-like structures that project from parapodia, modified into hooks called uncini which help anchor the worms in their tubes. The sedentary worms' heads typically have tentacles or feathery gills called branchiae, which are used for feeding and respiration. Morphological features of polychaetes include the structures of the prostomium and peristomium (the head and mouth regions), the shapes of the branchiae, parapodia, setae, and of internal features such as the proboscis, which is a reversible pharynx that errant polychaetes use for feeding. (JK 27)
Fan Worm (
Fan Worm (

A few species of polychaetes have a planktonic larval stage called a trochophore, which will eventually mature into a juvenile annelid. (SK,
external image image006.jpg

Class Hirudinea
The majority of this class includes leeches, which feed on invertebrates and are blood sucking parasites that attach temporarily to other animals. Hirudinea are characterized by a flattened body with reduced coelom and segmentation. They lack setae, and have suckers at anterior and posterior ends. Most of these organisms are parasites, predators and scavengers.(1) The majority of leeches are found in slow-moving, shallow freshwater, but a few species are found in moist soil or in the ocean. Leeches are mid-level consumers; some are blood-sucking and some are predatory. The former group feeds on mammals, fish, birds, or turtles and has chemicals in its saliva that prevents clotting. The latter group feeds on worms, snails, and insects and sometimes uses crushing jaws to kill its prey. (HB) (12)

A leech of the class Hirudinea (SG 26)

Blood Sucking Leeches (
Blood Sucking Leeches (

Acquiring and Digesting Food

The coelom of an earthworm is separated by septa, but the digestive tract run through the animal. The digestive system is separated into specialized regions. These regions are the pharynx, the esophagus, the crop, the gizzard, and the intestine. The pharynx is a muscular tube that extends from the mouth to the subpharyngeal ganglion. The esophagus moves food from the pharynx to the intestines. (1)

Most aquatic species scavenge or filter feed to ingest food, while land dwelling worms consume organic earth and digest the soil, pulling the nutrients out of the soil. (9)(TD) Digestion occurs in the body cavity, not in cells. Glands release digestive enzymes into the cavity, beginning the digestive process. The digestion is most active 18 hours after the food is ingested. (10)(TD)

Digestive System of an Earthworm (4)
Digestive System of an Earthworm (4)

Sensing the Environment
Annelids have a brain like pair of cerebral ganglia that lies above and in front of the pharynx. A ring of nerves around the pharynx connects to the subpharyngeal ganglion. From the subpharyngeal ganglion, nerve cords are attached and run the length of the worm. These nerve cords penetrate the segmenting septa to run the length of the worm, but to solve the issue of an encompassing nervous system, the nerve cords are fused with segmental ganglia. (1)

Annelids have six major types of sensory organs/structures, including statocysts (which aid in measuring equilibrium), eyes, palps, nuchal organs ( paired ciliated pit related to chemosensory activity) in the posterior of the brain, lateral organs (densely ciliated pits or papillae), antennae, and eyes. Some have "taste buds and tactile tentacles" Both the anntenae and palps are located the head-region of annelids and are used primarily to receive sensory information, although some palps can be used to aid feeding. The lateral organs are epidermal (skin) sensory cells that are sensitive to light and touch. Nerve cords also allow lateral nerves and ganglia to exist along the body. (MO, 19, 20)

Nervous System of an Earthworm (4)
Nervous System of an Earthworm (4)

Annelids primarily use setae to move. These are bristles that allow the worm to burrow through the ground by creating traction. In the case of Polychaeta ("many setae"), parapodia contain setae made of polysaccharide chitin. Earthworms, and other annelids, have a hydrostatic skeleton, which consists of fluid held under pressure in a closed body compartment. The coelomic fluid acts as the hydrostatic skeleton and is divided by septa between the segments of worm. This allows the worm to change the shape of each segment using longitudinal and circular muscles. The skeleton allows the worms to move by peristalsis, a type of locomotion that uses the rhythmic waves of muscle contractions from head to tail. (1)

Peristaltic Locomotion in an Earthworm (3)
Peristaltic Locomotion in an Earthworm (3)

An annelid’s respiratory organ is its skin. The skin contains blood vessels that can take in surrounding oxygen and send the oxygenated blood through the bloodstreams. In many Polychaetes, the parapodia are richly supplied with blood vessels and function as gills. Annelids usually rest in moist areas such as water and soil because their skin requires direct contact with water. (1)

Annelids have no "true respiratory organs" (as stated above, it is the skin): all respiration and gas exchange occurs in the capillary beds of the skin with the environment. Oxygen is carried with hemoglobin, and blood is collected into dorsal vessels. Along the esophagus, there are five pairs of aortic arches, which slightly resemble miniature hearts. (DZ 11)


Annelids have a closed off circulatory system, which means that blood is confined to vessels and is distinct from the interstitial fluid. Hearts pump blood into large vessels that branch into smaller ones. As blood passes through small vessels, chemical exchanges occur between the blood and the interstitial fluid. Materials are diffused between the blood and the interstitial fluid that surrounds the cells. In an earthworm, a dorsal vessel which acts as the main heart, and the two ventral vesselspump blood through peristalsis. 5 pairs of heart-like vessels loop around the digestive tract at the anterior end of the worm and connect the dorsal and ventral vessels. (1) The blood being pumped usually has hemoglobin, which is a red, oxygen carrying pigment. Some annelids however, have a green oxygen carrying pigment while others have unpigmented blood. Some polychaetes and leeches have partially open circulatory systems, where the blood and the coelomic fluid mix in the sinuses of the body cavities. (NY 5)

Circulatory System of an Earthworm (4)
Circulatory System of an Earthworm (4)
Closed Circulatory System (2)

Waste Removal System
In each segment of the worm is a pair of excretory tubes called metanephridia. These tubes contain ciliated funnels, known as nephrostomes that remove wastes from the blood and coelomic fluid. (1)
Annelids have tubular structures called nephridia that filter wastes out of fluids other than blood. They do this by using cilia or flagella to bring fluid into the tubular system, which leaves cells and proteins in the annelid's tissues. The nephridia reabsorb glucose, amino acids, and other substances that are useful to the annelid and return them to the tissues so that they can be used again. The nephridia also secrete excess ions into the fluid that was brought in by cilia or flagella. Any excess water, ions, and metabolic wastes leave the body through nephridopores in the body wall of the annelid. (AF 14).
Most aquatic annelids excrete ammonia, because it needs to be diluted, while terrestrial annelids, such as earthworms, excrete urea. Ammonia is usually excreted across the surface of the body. Some annelids also have a primitive liver-like tissue, called chloragogen tissue, which breaks down amino acids. (AM 8)
Excretory System of an Earthworm (4)
Excretory System of an Earthworm (4)

Self Protection
The hydrostatic skeleton protects an annelid's internal organs. Also, the fluid filled cavity known as the coelem maintains the coelmic fluid pressurewhich protects the inner organs. Worms are able to evade predators by burrowing themselves in the ground. They are highly resilient because if they are cut, the two halfs can grow back into 2 worms! (1) In harsh environmental conditions such as extremely cold, hot, wet, or dry soil, annelids must find ways to protect themselves. Usually, annelids dig deeper into the soil. Some species of annelids roll into balls and excrete mucous around themselves, forming a cocoon. This bodily state is called estivation. The annelid's body functions slow down and they wait for the soil conditions to improve. (7)(EH)

Osmotic Balance
An annelid’s skin can be used in gas exchange. However, the skin is susceptible to losing large amounts of water. Due to this, annelids are found in water and damp environments. (1)

Temperature Balance
Annelids cannot maintain a steady body temperature on their own. As a result, their respiration rate increases with increasing temperature and decreases with decreasing temperature. This is because increasing respiration releases more energy, due to an increased metabolic rate. (6) (MY)

Most annelids have separate sexes, but some can be hermaphrodites (Both male and female). Earthworms are hermaphrodites, but they can cross fertilize. Two worms mate by aligning themselves in a way they can exchange sperm. The received sperm cells are stored temporarily while the clitellum secretes a mucous cocoon. The cocoon slides along the worm, picking up eggs and the sperm. The cocoon falls off the worm’s head where it rests in the soil for development. They can also reproduce asexually with fragmentation, budding or fission.
Fission is a method that allows the worms to reproduce quickly and efficiently, with the body breaking into fragments to reproduce. The posterior part of the body breaks off and forms a new individual. Although some other taxa have different abilities to give rise to new amputated segments, many do not reproduce this way (MSaraf 15). The video bellow shows two earthworms mating. (1) Tail segments regenerate more readilly than head segments, and there seems to be a limited number of times regeneration of segments can occur. This number, however, varies from species to species. There is no known species that will form two worms when cut in half (RS 6).

ARKive video - Earthworms mating

Review Questions

1. What are the three main classes of annelids and what are the differences between them?(AK)
2. How is a hydrostatic skeleton favorable for annedlids? (NS)

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