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Annelida is a class of animals also known as segmented worms or ringworms. The phylum is
large, having over 17,000 different species. Throughout the 20th century Annelida was divided into the
groups; Polychaeta, Oligochaeta (earthworms etc.) and Hirudinea (leeches). Generally, theythrive in
marine and fresh waters; mostly in sediments and in damp terrestrial environments; mostly the soilThe
familiar annelids are the earthworms and leeches. However, the polychaetes are the most dominant of
the phylum and are found in almost all marine environments. There are about 8,000species of
Polychaetes which include rag worms, lugworms, bloodworms, sea mice, andmuch more (Polychaete,
2019). Few organisms within this group also occur in fresh water and moist terrestrial surroundings.
There are also pelagic polychaetes which swim and prey on plankton (Phylum Annelida, n.d; Rouse et
al., 2001). There are about 3,500 species of oligochaetes. They live in freshwater, salt water and in soil.
Oligochaetes are known to be important recyclers in the environment. For example, the earthworm
which makes soil. This class of Annelid range in length from a few millimetres to over 10 feet
(Hirudinea, 2019). The Leech, which is a subclass of Hirudinea, has about 650 species. They are known
for having sucker. The length of their bodies ranges from a few millimeters to about 20 cm or even
longer when the animal extends to stretch. These organisms thrive in fresh water and on land (Friedl,
2019).Species from this Phylum possess unique shapes and colours. In addition, annelids belong to the
invertebrate group. These organisms are coelomate and triploblastic, with bilateral symmetry (Phylum
Annelida, n.d.; Rouse et al., 2001).
All annelids have a worm shaped segmented body. Annelid structure and function (2019) states
that the number and organization of the bristles and appendages differ also they also protrude away from
the body. Reish (n.d) included that there are many variations such as the tentacles and formation of the
feeding tentacles on the animals, some of the tentacles are extremely elaborated and causes the animals
to look unlike that of a worm. Annelid structure and function (2019) also spoke about its body symmetry
and included that they are bilaterally symmetrical animals and ranges in size from less than 1mm in
length to more than 3m. Reish (n.d) claimed that he width of annelids may also exceed about 2.5 cm
when in its contracted state and usually has a fluid filled cavity between the outer body wall and the gut.
Reish (n.d) indicated that annelid’s bodies are covered by an external cuticle that never sheds or molts,
and the chaetae which is found within the body has a cuticular structure and contains large amounts of
chitin. Annelid structure and function (2019) also included that there are longitudinal muscles which are
present as four distinct bands and the brain is also another part that varies in terms of its structure.
• There are various characteristics that differentiate Annelida from all otherphylum.
• Annelid bodies are built up from large numbers of similar segments arranged onebehind the
• Most of them can be found in marine, streams, freshwater pools and terrestrialhabitats.
• Their bodies are bilaterally symmetrical and segmented
• Every segment is separated from contiguous ones by a transverse septum
• Annelids body is covered with a flexible non-chitinous cuticle. It is alsotriploblastic with a well-
• Most of them have forms that have setae, hard, chitinous structures.
• Their digestive system is complete and they also have a closed circulatorysystem.
• Their excretory system typically consists of a pair of nephridia for each segment.
• They are free-living and mostly marine
• Many setae and lateral outgrowths of the body wall known as parapodia
• They have well-developed head bearing appendages
• Sexes are separate and they are free-swimming trochophore larva.
• They are freshwater species and are terrestrial
• They do not have a distinct head
• Appendages eggs are deposited in a cocoon and development is direct
• Clitellum is present and no parapodia is found
• Most times they are dorso-ventrally flattened with fixed number of body segments(34)
• Clitellum present only during reproduction
• No setae present or parapodia
• They are Ectoparasites but not all of them, most of them feed on organic debrisand some of them
are fluid feeders
• Two suckers are found within the leech’s mouth both anterior and posterior.
Friedl (2018) claimed that regarding reproduction the phylum Annelida varies in reproduction
depending on the species or class. Members of Annelida could possibly reproduce sexually or asexually.
Asexual reproduction does not involve the formation of gametes (eggs and sperm), and it usually occurs
either by budding or fission.
Friedl (2018) stated polychaetes reproduce asexually by fission, when the worm makes an exact
copy of its DNA, cloning itself into a new individual. Another form of asexual reproduction is budding.
Where a smaller fragment breaks off with the same DNA as the 'parent' worm and becomes a new
individual. In budding, the new individual is a smaller worm that will grow and develop into an adult
An annelid undergoing asexual reproduction by budding.
Mivart, G. (2007). [Annelid undergoing spontaneous fission]. Retrieved from
Harris (2019) wrote some polychaetes reproduce sexually, depending on the species. Organisms
under this class can have separate sexes or can be hermaphroditic. In sexual reproduction, these marine
worms physically change where several of posterior segments converts to reproductive segments
containing ovaries or testes. The digestive tract also degenerates, and the worm develops enhanced
appendages for swimming to the ocean surface. According to Harris (2019), eggs and sperm are both
released into the open water surface, where external fertilization occurs. The externally fertilized
zygotes go on to develop into a specialized larval form called a trochophore larva. After a period, the
larvae settle to the sea floor and undergo metamorphosis to the juvenile form and further develop into
adult worms. She also stated this form of reproduction is called epitoky and there are several variations
seen in different polychaete species
Campbell (2018) wrote; the remaining classes of Annelida: Hirudinea and Oligochaeta are
simultaneous hermaphrodites, having both male and female reproductive organs. However, they do not
self-fertilize. Instead they reproduce sexually. She stated leaches intertwining their bodies together when
mating and release a spermatophore which attaches to the other leech. Once attached, the sperm exits the
spermatophore, through the skin of the other leech traveling to the ovaries to fertilizes the eggs.
Freidl (2017) include during sexual intercourse among Oligochaeta, both sets of sex organs are
used by both worms where the eggs of both mates become fertilized. When attaching to each other, they
do it in the opposite direction.
Campbell (2018) also stated, both classes make cocoons to deposit their eggs. The cocoons act as
protective and nutritive barrier. When the worms hatched, they appear as smaller versions of the
‘mother’ worms. The new worms will later develop and growth greatly to an adult.
According to Wilkin, P. D., & Blanchette (2017), the phylum annelids comprises of many
niches. Some, like the leeches and marine worms, are parasitic while others filter-feed or prey on
invertebrates. However, the most significant ecological role played by annelids is the reworking of soil
and sediments. Terrestrial annelids such as the earthworms, borrow themselves in the ground which
helps to loosen the soil and allows oxygen and water to penetrate in while others live entirely on the
surface in moist leaf litter. However, both are beneficial since they help to produce soil by mixing
organic and mineral matter. Furthermore, Wilkin, P. D., & Blanchette (2017) noted that earthworms also
help to concentrate and convert minerals to forms that plants can use more easily and they assist in
accelerating the decomposition of organic matter which in turn makes it more quickly available to other
organisms. Earthworms are a form of food to birds as well, ranging in size from robins to storks, and for
mammals ranging from shrews to badgers.
As for marine annelids, Scaps (2002) stated that they make up over one-third of bottom-dwelling animal
species around coral reefs and in tidal zones. Burrowing species increase the penetration of water and
oxygen into the sea-floor sediment, which allows for the the growth of populations of aerobic
bacteria and small animals alongside their burrows.
On the other hand, Scaps (2002) explained that although blood-sucking leeches do little direct harm to
their victims, some transmit flagellates that can be very dangerous to their hosts.
The phyla of Annelida are considered by biologist to have many pivotal roles that assist in
human being's practices in many ways, whether it helps positively or negatively. Tanwar (2016),
elaborated that Annelids are most beneficial, in the agricultural economy of the world as they tend to to
be helpful to farmers. The author further explained that earthworms acts as a natural way of ploughing
the soil when they burrowing themselves inside and around the earth. This mechanism allows for the
aeration and up turning of the subsoil within the earth to the surface. Tanwar (2016) stated that this
causes the soil to be less saturated, have readily available nutrient contents and have better moisture and
air retention, so that plants can comfortably grow. The author also mentioned that their faeces matter can
also serve as a form of fertilizing manure, as its content is made up of nitrate, phosphorus, calcium and
even potassium, which are all good components of humus, that further assist in the growth of plants. It
was also mentioned by Shah (n.d), that earthworms are used as forms of decorative bait to attract fishes
and feed aquarium fish, frogs and birds.
Further importance of Annelids can be seen as food to some uncivilized people. Tanwar (2016),
expounded that the Pablo worms are used by people who practices native traditions as a source of daily
food consumption. Tanwar (2016) went on to express that these worms are considerably rich in
nutrients, which nourishes growth.
Annelids over the years have been used for medicinal purposes. Shah (2016) stated that, therapy
systems would utilize various types of Annelids to produce medicine that assisted in curing bladder
stones, rheumatism, issues with sexual impotency and other illnesses. According to Tanwar (2016),
countries like China, India and Japan, uses leeches in the extraction of unwanted blood from the body,
which helps reduce swelling and discoloration in a painless manner to the patient. Tamara (2016) further
explained that a valuable substance called Hirudin, which is taken from leeches, are used as an
anticoagulant, to prevent the formation of blood clots within the blood streams of human.
McGuffin (2017) stated that most annelids, such as the earthworm, feed on organic matter
consisting of mainly dead plants and animals. Additionally,as they borrow through the earth, they ingest
small particles of soil and excrete their waste material in the form of casts. McGuffin(2017) further went
unto explain that these castings are high in nutrients such as nitrogen, phosphorous and potash. The Cool
Galapagos (n.d) further noted that some annelids consume small invertebrates for instance. In the case of
leeches, they dwell in tubes and feed by grabbing their prey with their jaws. The blood leeches however,
feed through suckers found on both ends of their body. Cool Galapagos explained that they secrete an
anti-coagulant called hirudin which reduces blood clotting so as to allow easier feeding while on the
other hand, feather duster worms use radioles (feather like tentacles) to capture plankton and small
According to Battista (2019), unlike other organisms there is low requirement for the very
specialized digestion system in the annelid’s body structure however it contains all the indistinguishable
organs. Baxamusa (2019), mentioned that these organs are from the mouth to the anus which also
comprises of the mouth, buccal cavity, pharynx, esophagus, crop, gizzard, intestine and anus. However
regardless of all these present organs, the processing happens through a long cylinder that reaches out
from the mouth to the anus which is found in the fontal of the body structure throughout. The passage is
unsegmented not normal for the worm’s outside structure making it so identical to a human’s digestive
Baxamusa (2019) also added that annelids are officially lacking teeth and jaws in the body
structure so instead of eating, their bodies functions to mash their intake of soil or animal matter in the
gizzard. After foods are intake into the mouth, it makes a trip down to the pharynx and passes it through
the throat (esophagus). Just like human’s stomach, the food gets stored in the yield before it gets crushed
up in the gizzard. The gizzard however is the last area the food is led to before the intestine (Battista,
The process moves on just the same as human’s, the digestive tract contains synthetic chemicals
that breakdown the nourishment for the organism, retaining the supplements through the digestive tract
divider also known as the intestinal wall. When it takes all the nourishments that it needed from the
food, the rest is discharged through the anus, excreting the food that is left to be known as waste
Diagram of the Digestive System
(Phylum Annelida: An Overview of Biodiversity, n.d.).
Phylogenetic Tree Showing the Derivative of the Annelida Phylum
C n id
a ri a
s c a
p o d
E c h
in o d
C h o
N e m
Bilateral symmetry Radial
Classes of Animals found within the Annelida Phylum
Three (3) Classes of animals found within the Phylum: Annelida (University of Edinburgh, 2007)
• Oligochaeta (the earthworms)
• Polychaeta (the marine worms)
•Hirudinea (the leaches)
Nguyen, A. (2018). [Photograph of Earthworm].
Retrieved from https://www.pinterest.com/pin/
Leonard, W. (2005). [Photograph of Oligochaeta].
Retrieved from https://calphotos.berkeley.edu/cgi
[Photographs of Hirudinea in its habitat]. (2018). Retrieved from https://twitter.com/lhirudinea
Watanabe, J. (2004). [Photograph of Polychaeta].
Retrieved from https://www.pinterest.com/pin
[Photograph of Phylum Annelida]. (2017).Retrieved
Annelid structure and function. (2019).Retrieved from: https://www.ck12.org/book/CK-12-Biology-
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Baxamusa, B. N. (2019). Digestive System of an Earthworm. Retrieved from:
Campbell, M. (2018). Life Cycle of Leeches. Retrieved from: https://sciencing.com/life-cycle-leeches-
Cavendish. M (2007) Animal and Plant Anatomy, Vol 2 ed, Tarry Town, New York: Marshall
Cool Galapagos. (n.d.). Invertebrates: Annelidia- Annelids-Segmented Worms. Retrieved from:
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Friedl, S. (2019). Oligochaeta: Characteristics & Reproduction. Retrieved from:
Harris, L. (2019). Polychaetous Annelids. Retrieved from: https://nhm.org/site/research-
Hirudinea. (2019). Retrieved from https://www.britannica.com/animal/leech
Leonard, W. (2005). [Photograph of Oligochaeta]. Retrieved from https: //calphotos. Berkeley.edu/cgi/
Nguyen, A. (2018). [Photograph of Earthworm]. Retrieved from https://www.pinterest.com/pin/ 511 9
McGuffin, D. (2017). Pets: What food do earthworms eat?. Retrieved from: https://animals.mom .me/
Phylum Annelida. (n.d.). Retrieved from: https://www.toppr.com/guides/biology/animal-
Phylum Annelida: An Overview of Biodiversity. (n.d.). Retrieved from:
Polychaete. (2019). Retrieved from: https://www.britannica.com/animal/polychaete
Reish. (n.d). Annelid: Invertebrate. Retrieved from: https://www.britannica.com/animal/annelid
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Shah, R. (n.d). Economic Importance of Earthworm. Retrieved from: http://www.biologydiscussion.com
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Wilkin, P. D., & Blanchette, J. (2017). Annelid Ecology. Retrieved from: https://www. ck12 org/biology