Annelids are the segmented invertebrate worms belonging to Phylum Annelida in the Animal kingdom. Annelids are successful in terrestrial and aquatic habitats. Their terrestrial inhabitance demands a moist and humid environment. Phylum Annelida is large and diverse with bilaterally symmetrical triploblastic coelomate invertebrates. The phylum includes filter feeders, predators and blood−sucking parasites also. The members of Phylum Annelida are referred to as Annelids. Most annelids adopt a parasitic or commensal mode of life in their respective environments. Their body sizes range from millimetres to 3 meters. The phylum has its own morphological features which mark the annelids distinct from the other organisms of the animal kingdom.
The word Annelida is derived from the Latin term Annelus meaning small ring and eidos meaning form. The name represents their body structure. The word Annelida was first coined by Lamark
The general characteristics of phylum Annelida are−
Vertebrates include the following classes−
Bilaterally symmetrical, triploblastic (presence of three germ layers), eucoelomates (with a true body cavity).
Body organisation is organ level.
Long segmented bodies with the tube within the tube structure. The ectoderm comprises muscles, epidermis and cuticle. Within this is a coelomic cavity that encloses the inner tube of the digestive tract
The body is segmented metamerically. Metamerism is the evolutionary developmental stage where organisms show the body segments called metamers. Annelids are the first organisms in the Animal kingdom to exhibit metamerism
Setae and parapodia function as locomotory organelles. They repeat for each segment. Setae or chaetae are bristle−like and composed of chitin. They are epidermal structures clearly visible on the annelid body. Parapodia are paired protrusions from the body. Some species have chaetae directly on the skin or while some have them on parapodia
Annelids have a special reproductive structure called the clitellum. It is an unsegmented thickened part located near the head region. Clitellum has a glandular function and is associated with the secretion of viscous fluid necessary for cocoon formation. Clitellum also secretes mucus needed for the copulation in annelids
The segmentation in Annelids is both external and internal. The external segmentation forms ring−like annuli while the internal segmentation is maintained by transverse septa. Each metamer has its own set of organs while the digestive tract is long and continues linearly through the metamers.
The body wall contains the outer circular muscles and inner longitudinal muscles
The digestive system is completely developed. The segmented trunk (prostomium) has a mouth on the first segment (peristomium) while the pygidium contains the anus
The food is ingested through the mouth and passed to the crop through the oesophagus by peristaltic movement. The crop stores food before letting it into the gizzard. Gizzard mashes the food and passes it into the intestine where digestion and absorption take place. Typhlosole is a special folded region facilitating the absorption in the intestine. The undigested waste is excreted out of the anus
A specialized respiratory system is absent and respiration is carried by the moist skin. Beneath the skin lies the capillaries that carry oxygen to body parts and deliver carbon dioxide to be expelled out of the body.
The circulatory system is closed. Blood flows through dorsal and ventral blood vessels which run longitudinally along the segments. There are small capillaries that function in each segment. The anterior metamers contain muscular pumping hearts emphasising that annelids have a well−developed circulatory system
In most annelids, the brain is simple and is a mass of nerves that coordinate the organism's movements according to light, touch and temperature. There is a single or double nerve cords running longitudinally along the length of the annelid body till the tail and within each segment are ganglion and nerves. Some annelids like earthworms have a complicated brain with an extra ganglion below the pharynx. It is called the sub−pharyngeal brain
Excretion is performed by paired nephridia which collect the coelomic fluid through a ciliated nephrostome. The nephrostome opens into a central canal and terminal vesicle. The excretory wastes and excess water are finally discarded out of the body through the nephridiopore that lies on the ventral side of the body.
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Phylum Annelida is divided into the following classes.
Polychetae
Oligochetae
Hirudinea
Archiannelida
Although the general characteristics like soft invertebrate and segmented bilaterally symmetrical bodies are the same, the typical characteristics vary and are tabulated.
| Character | Polychetae | Oligochetae | Hirudinea |
|---|---|---|---|
| Habitat | Marine | Freshwater, terrestrial | Parasites in freshwater, marine and terrestrial |
| Lifestyle | Sedentary inhabiting burrows and active predators too. | Mostly filter feeders | Blood sucking parasites with a special sucking organ called a proboscis |
| Setae | Many chitinous setae | Few and small hardly visible setae | Absent |
| Parapodia | For respiration and locomotion | Absent | Absent |
| Clitellum and cocoon formation | Absent | Present | Present |
| Fertilisation | External | Cross fertilisation and external | Internal |
| Development | Indirect with trochophore larvae | Direct | Direct |
| Examples | Ragworms | Earthworm | Leeches |
Archiannelids possess many reduced annelid characteristics. They inhabit marine environments and lack setae and clitellum but some consider them as ancestors of annelids. Their nervous system is primitive. Parapodia may be present but the body lacks external segmentation. The development is indirect with trochophore larvae. Examples are Nerilla and Dinophillus.
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Annelids reproduce both sexually and asexually based on the species.
Asexual reproduction is by fission or budding. Annelids show high ability in regeneration
Sexual reproduction is the fusion of male and female gametes
In some annelids, the male and female sexual organs are in separate bodies. The marine polychaetes undergo a reproductive transformation (digestive system degenerates and locomotory appendages develop) and reach the ocean surface to release their gametes and organisms die off
Some species are hermaphrodites (both male and female in the same organisms) and cannot self−fertilise. The sperms are stored in spermatheca and when two organisms mate, they align their bodies so that they exchange the sperm into a cocoon which is secreted by clitellum. Within the cocoon, they deposit the eggs and sperms from the mating partner and fertilisation takes place inside the cocoon
Fertilisation is internal in leeches which discharge the cocoon after fertilisation from which the offsprings develop
Annelids are heterotrophs and depend on dead organic material or other organisms as the source of nutrition
Marine annelids are filter feeders, predators, and scavengers depending on the species. The filter feeders have special palps containing ciliated crowns that allow the filtering of feed entering into the mouth. The predators are equipped with jaws and eversible pharynx both of which are used for grasping and ingesting prey.
Annelids like earthworms are soil dwellers and feed on dead organic material as a source of nutrition
Leeches are aquatic parasites and are also capable of living in terrestrial habitats. They have dorsoventrally flattened bodies with special suckers on both the body ends. The specialised proboscis facilitates the predation on small prey. The suckers and special chemical hirudin (anticoagulant) facilitate parasitic living
Annelids like lugworms are sedentary forms settling in tubes or burrows in the sand or on rocks
Phylum Annelida is large and diverse with soft and segmented−bodied invertebrates. Annelids are triploblastic Euceolomates inhabiting the terrestrial as well as aquatic habitats. Annelids perform cutaneous respiration and hence most of them live in moist environments only. The presence of setae and parapodia are special characteristics of the annelids. The body segments are called metamers and each metamer contains organs necessary for body function. Annelids are further divided into Polychaetes, Oligochaetes, Hirudineans, and Archiannelids. Reproduction is either asexual or sexual type based on the species. The clitellum is a special glandular structure necessary for sexual reproduction. Annelids have great ecological as well as evolutionary significance
1. What are the special characters of leeches?
Ans: Leeches are blood−sucking parasitic annelids. They have special suckers and anticoagulant hirudin in saliva. The body segments are 32.
2. What is vermicompost?
Ans: Vermicompost is the production of manure from organic material using earthworms
3. What is the significance of earthworms?
Ans: Earthworms feed on the dead decomposed organic matter, adding nutrients to the soil. Their soil−dwelling behaviour helps to maintain the drainage in soils and keeps the soil structure high.
4. What are the dorsoventrally flattened annelids?
Ans: Hirudineans are dorsoventrally flattened annelids.
5. Do Annelids have the ability to regenerate?
Ans: Regeneration is common among a few animal phyla. In many, the regeneration ability is confined to tail parts. Annelids have a specialised ability to regenerate the lost tail parts as well as head segments depending on the number of segments lost.
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