Description of Phylum Annelida, Study notes for Biology

Description of Phylum Annelida, Study notes for Biology

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Phylum Annelida


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.

Distinguishing characteristics

• 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-

developed coelom.

• 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.

(Cavendish, 2007).

Specific Characteristics


• 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.

(Cavendish, 2007).


• 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

(Cavendish, 2007).


• 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.

(Cavendish, 2007).


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

sized polychaeta.

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.

Economic Importance

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

floating organisms.


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

(Battista, 2019).

Diagram of the Digestive System

(Phylum Annelida: An Overview of Biodiversity, n.d.).

Phylogenetic Tree Showing the Derivative of the Annelida Phylum

P o

ri fe


C n id

a ri a

P la

ty h

e lm

in th


s M

o llu

s c a

A n

n e

lid a

A rt

h ro

p o d


E c h

in o d

e rm

a ta

C h o

rd a


N e m

a to

d a







No body



Bilateral symmetry Radial


True Tissues



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)

Class Oligochaeta

Nguyen, A. (2018). [Photograph of Earthworm].

Retrieved from


Leonard, W. (2005). [Photograph of Oligochaeta].

Retrieved from



Class Hirudinea

[Photographs of Hirudinea in its habitat]. (2018). Retrieved from

Class Polychaeta

Watanabe, J. (2004). [Photograph of Polychaeta].

Retrieved from


[Photograph of Phylum Annelida]. (2017).Retrieved




Annelid structure and function. (2019).Retrieved from:


Battista, J. (2019). Phylum Annelida Digestive & Respiratory Systems. Retrieved from:


Baxamusa, B. N. (2019). Digestive System of an Earthworm. Retrieved from:

Campbell, M. (2018). Life Cycle of Leeches. Retrieved from:


Cavendish. M (2007) Animal and Plant Anatomy, Vol 2 ed, Tarry Town, New York: Marshall

Cavendish Corporation.

Cool Galapagos. (n.d.). Invertebrates: Annelidia- Annelids-Segmented Worms. Retrieved from:

Freidl, S. (2018). Annelid Reproduction: Facts & Example. Retrieved from:


Friedl, S. (2019). Oligochaeta: Characteristics & Reproduction. Retrieved from:

Harris, L. (2019). Polychaetous Annelids. Retrieved from:


Hirudinea. (2019). Retrieved from

Leonard, W. (2005). [Photograph of Oligochaeta]. Retrieved from https: //calphotos.


Nguyen, A. (2018). [Photograph of Earthworm]. Retrieved from 511 9

3268 2617195710/?lp=true

McGuffin, D. (2017). Pets: What food do earthworms eat?. Retrieved from: .me/


Phylum Annelida. (n.d.). Retrieved from:


Phylum Annelida: An Overview of Biodiversity. (n.d.). Retrieved from:

Polychaete. (2019). Retrieved from:

Reish. (n.d). Annelid: Invertebrate. Retrieved from:

Rouse, G., Fredrik, P., & Damhnait, M. (2002). Annelida. Retrieved from:

Scaps, P. (2002). Springer Link: Annelida: Polychaeta. Retrieved from:


Shah, R. (n.d). Economic Importance of Earthworm. Retrieved from:


Tanwar, H. (2016). Annelida: Economic Importance. Retrieved from: http://anelidaeconomicimp.blogs

University of Edinburgh. (2007). Phylum Annelida. Retrieved from:


Wilkin, P. D., & Blanchette, J. (2017). Annelid Ecology. Retrieved from: https://www. ck12 org/biology


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