Annelids, commonly known as segmented worms, are soft-bodied, bilaterally symmetrical invertebrates of the phylum Annelida. They constitute the earthworms, leeches, and ragworms, traditionally grouped into polychaetes, oligochaetes (including earthworms), and leech-like species.
The phylum derives its name from the Latin ‘annelus,’ meaning ‘little ring,’ which refers to the continuous, ring-like segments (annuli) of their body that are often separated by wall-like septa. Each segment has the same set of organs as other invertebrates, while some groups also possess parapodia (lateral outgrowths) for locomotion.
Approximately 22,000 species of annelids exist and have adapted to live in all habitats, including marine, freshwater, and terrestrial environments.
Annelids usually range between an inch and about 20 feet in length. However, the largest annelid, the African giant earthworm (Microchaetus rappi), grows up to 22 feet long, and the Australian giant Gippsland earthworm and Mekong giant earthworm (Amynthas mekongianus) measure around 10 feet.
While free-moving polychaetes, like tubeworms, and oligochaetes, like earthworms, constitute some of the largest species, leeches measure an average of 15 inches.
An annelid’s body is segmented into a linear series of rings called annuli. Each segment contains the same set of organs (metamerism), though they share a common gut, as well as nervous and circulatory systems. The body is also covered by a tough but flexible cuticle composed of collagen.
The bodies of annelids are divided into identical segments containing the same sets of internal organs, external chaetae (chitinous bristles), and, in some species, appendages. The frontmost segment, the prostomium, houses the brain and sense organs, whereas the rearmost, the pygidium or periproct, contains the anus. Since both the segments are compositionally different, they are not considered true segments.
In polychaetes, the peristomium, the segment just behind the prostomium, features chaetae and appendages. However, some experts do not classify these appendages as true segments.
The body segments undergo teloblastic growth, developing one at a time from a growth zone in front of the pygidium. Thus, the youngest segment lies just before the growth zone, whereas the peristomium is the oldest, lying in the extreme front.
The external layer of an annelid’s body comprises a cuticle composed of interwoven collagen fibers. These fibers are produced by the outermost skin layer or the epidermis; however, some annelids (like those living in underwater tubes) completely lack them.
The vascular dermis is the next layer under the epidermis, characterized by a strong network of connective tissues and proteins like collagen. Below the dermis lie two muscular layers (circular and longitudinal) that develop from the body cavity or coelom. Several annelid families, however, have lost the circular muscles.
A series of hair-like projections called setae extend from the epidermal layer and help in movement along a substrate.
These invertebrates possess limb-like, unjointed lateral outgrowths called parapodia that often possess comb-like chaetae at their tips. These tiny structures, similar in appearance to the setae (but compositionally different), are made of the flexible protein β-chitin.
Chaetae have follicles that include hair-forming chetoblast cells and muscles that help project and retract them. The chetoblasts produce microvilli (fine, hair-like extensions) that increase the surface area of the chaetae.
In active swimmers, like sludge worms, the parapodia are divided into large upper and lower paddles, sometimes with cirri (tufts of fused cilia) and gills distributed over them.
Annelids with well-developed septa possess blood vessels all along their body and below the gut. A network of capillaries in the body wall transports blood through the blood vessels to deliver oxygen and nutrients to the required segments.
Two major blood vessels, the upper and the lower, pump blood throughout the body by contracting. In some species, the anterior end of the upper vessel enlarges into a heart, whereas in many earthworms, the accessory vessels that connect the upper and lower main vessels function as hearts.
In contrast, annelids without distinct segmentation, such as spoon worms, lack blood vessels and rely on coelomic circulation to deliver oxygen and nutrients.
Although gaseous exchange usually occurs through the skin, polychaetes often respire through gill filaments, which are extensions of the parapodia in their body segments. In some aquatic oligochaetes, like the sludge worm, respiration occurs through the rectum.
In annelids, oxygen transport is facilitated by a respiratory pigment dissolved in the plasma. Most polychaetes and oligochaetes use hemoglobin for this purpose. However, some other groups use a different pigment, green chlorocruorin, to carry oxygen in their blood.
In polychaetes, the gut is almost straight and is associated with the vertical partitions between the septa called mesenteries. The mouth opens to an eversible pharynx, followed by an esophagus, then the intestine, and finally, the anus located on the underside of the pygidium.
In oligochaetes, such as earthworms, the digestive system starts with the mouth, leading to a muscular pharynx, followed by the esophagus and a muscular gizzard, which helps in grinding food. Almost all annelids, except for leeches, have poorly developed diverticula that function as digestive glands.
In the family Siboglinidae, the gut is reduced and contains a swollen lining that houses symbiotic bacteria. These bacteria account for approximately 15% of the annelid’s total body weight and play a crucial role in their digestion.
Annelids are typically ammonotelic, excreting ammonia through metanephridia or protonephridia, although earthworms excrete urea. Metanephridia are funnel-shaped, ciliated structures, while protonephridia are flagellated tubules that open outside via a duct.
Both excretory systems use a two-stage filtration process. In protonephridia, both the stages occur within the organ itself. In metanephridia, the initial filtration is aided by special filter cells in the blood vessel walls, allowing small molecules and fluids to pass into the coelomic cavity and enter the metanephridia. Common to both systems, the second filtration stage involves reabsorbing useful metabolic substances from the filtered fluid and waste.
The central nervous system of annelids consists of a ring-like brain surrounding the pharynx and a ladder-like ventral nerve cord, which includes a pair of nerve cords running along the body’s underside. The nerves are characterized by giant axons that enable quick signal transmission.
Each body segment features paired ganglia interconnected by transverse bridges.
In polychaetes, the brain is typically located in the prostomium, whereas in clitellates, it is found in the peristomium or, occasionally, the first segment behind the prostomium. In highly mobile polychaetes, like goniadids (family Goniadidae), the brain is divided into a distinct forebrain, midbrain, and hindbrain.
Polychaetes have chemosensory paired ciliated structures called nuchal glands around their necks. They may also have combinations of simple eyes (ocelli) and compound eyes (camera eyes), which work together to form images.
Some burrowing and tube-dwelling polychaetes, like lugworms, possess statocysts for balance, while others have sensory palps on the underside of their heads.
Although the scientific views on the classification of the phylum Annelida keep changing, the traditional classification scheme is commonly followed. Based on this scheme, it is divided into the following groups.
Some other groups, earlier considered members of other animal phyla, are now considered annelids.
Phylogenomic studies have also suggested that members of the phylum Orthonectida and Nemertea are also annelids.
These invertebrates are found in various habitats, including damp soils, marine environments (such as seas, oceans, tidal zones, and hydrothermal vents), and freshwater bodies like rivers, lakes, and ponds.
Polychaetes float, burrow, or wander the ocean floors, whereas oligochaetes, like earthworms, are terrestrial and dig into the soil. Leeches live in both humid soil and freshwater environments.
Annelids feed on soil, tiny invertebrates, like nematodes and rotifers, and dead, decaying organic matter.
It was initially believed that all annelids are gonochoric, with distinct male and female individuals reproducing sexually.
While most polychaetes remain unisexual throughout their lives, others are either hermaphroditic (have both male and female organs in a single individual) or undergo sex reversal (alternate between male and female).
In around 25% of polychaete species, the males and females release their gametes (sperm and eggs) into aquatic environments through their nephridia. The females then collect the sperm and externally fertilize their eggs, although, in some species, males inject sperm using their penis. The fertilized eggs develop into planktonic, ciliated trochophore larvae.
These larvae eventually settle on the ocean floor and undergo metamorphosis into adults. During this process, the trochophore’s ciliated apical tuft and prototroch transform into the prostomium, while the area around the anus becomes the pygidium. The narrow band just before the pygidium develops into the growth zone, and the remaining body becomes the peristomium.
However, around 14% of species produce yolk-rich eggs, thereby eliminating the need for a larva. The remaining 61% of species take care of their fertilized eggs until they hatch. While some stick the jelly-covered eggs to their bodies, others keep them within.
Although most polychaetes breed continuously through multiple breeding seasons, some, like the pile worm, breed once in a lifetime.
Most oligochaetes and leeches are hermaphrodites, possessing both male and female sex organs within a single individual. Some leeches switch from male to female upon reaching maturity, while both have well-developed gonads.
In earthworms, sperm is stored in the spermatheca, a specialized organ for storing sperm. The clitellum produces a cocoon that collects eggs from the ovaries and sperm from the spermatheca, thereby facilitating fertilization. The fertilized eggs then develop directly into juveniles within the cocoon without passing the larval stage.
In leeches, fertilization occurs within the ovaries, and the fertilized eggs then move to the cocoon for development.
Polychaetes sometimes reproduce asexually by budding or by dividing into two separate parts.
In contrast, oligochaetes always reproduce asexually through division rather than budding. Some, such as Aulophorus furcatus, reproduce asexually throughout the year, while others do so only in the summer and autumn. Leeches, however, do not reproduce asexually at all.
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