Phylum Annelida

Introduction

The kingdom Animalia contains the phylum Annelida, a very broad phylum. Annelids can be found both in aquatic and terrestrial environments. These invertebrate organisms are bilaterally symmetrical, and their segmented bodies set them apart from other organisms, also known as the ringed worms or segmented worms. They are a large phylum, with over 15,000 extant species, including ragworms, earthworms, and leeches. The species have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in freshwater, and others in moist terrestrial environments.

Annelids are invertebrates that are bilaterally symmetrical, triploblastic, and coelomate. They also have parapodia for locomotion. The traditional classification of polychaetes (almost all marine), oligochaetes (which includes earthworms), and leech-like species is still used in most textbooks. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and polychaetes as a sub-group of polychaetes. Pogonophora, Echiura, and Sipuncula, previously considered separate phyla, are now polychaete sub-groups. Annelids belong to the Lophotrochozoa, a protostome “super-phylum” including mollusks, brachiopods, and nemerteans.

Annelids 

Annelid, also known as a segmented worm, is any member of a phylum of invertebrates distinguished by the presence of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they derive their name. Annelida is a phylum that includes segmented worms. These animals can be found in marine, terrestrial, and freshwater environments, but the presence of water or humidity is critical for their survival, especially in terrestrial environments.

The phylum’s name is derived from the Latin word annulus, which means “small ring.” This phylum’s animals form parasitic and commensal symbiosis with other species in their environment. In the phylum Annelida, approximately 15,000 species have been described. This phylum includes earthworms, polychaete worms, and leeches. Annelids exhibit protostome development in embryonic stages and are frequently referred to as “segmented worms” due to their key feature of metamerism, or true segmentation.

Annelida Phylum (Segmented Worms)

The body wall is coated by a cuticle secreted by the epidermal cells and contains an outer spherical and inner longitudinal muscle layer; chitinous (tough, carbohydrate material) setae are normally present, secreted by follicular cells and organised segmentally; Shuttered circulatory system, with blood frequently containing a respiratory pigment; coelom, of Schizocoelic origin, divided segmentally into compartments by walls, or septa; nervous system includes a dorsal, bilobed brain and a pair of connective nerves that encircle the digestive tract and unite to form a ventral nerve cord with one ganglion per segment.

Compared to polychaetes, the number and size of chaetae in Oligochaetes are greatly reduced (poly=many, chaetae = hairs). Polychaetes’ many chaetae are also organised within white flesh, flat, paired appendages that protrude out of each series called parapodia, which may be specialised for various functions. Hirudinea is a subclass of leeches that includes Hirudo medicinalis and Hemiclepsis Marginata.

Hirudinea and Branchiobdella are subclasses of the class Oligochaeta. Leeches are distinguished from other annelids by the development of suckers at the anterior and posterior ends of their bodies and the absence of chaetae. Furthermore, the segmentation of the body wall may differ from the internal segmentation of the coelomic cavity. Because leeches consume a lot of blood from their hosts, this adaptation may help them live longer. Branchiobdella Balcanica Ski and Branchiobdella Astaci are subclass Branchiobdella worms linked to leeches and oligochaetes.

Polychaeta

• The body is elongated and divided into segments.
• They are found in the marine environment.
• These are true coelomates, bilaterally symmetrical worms.
• They excrete through metanephridia and protonephridia.
• Fertilisation is external.
• They have a well-developed nervous system.
• The circulatory system is closed type.
• They are hermaphrodites.
• They might possess fin-like appendages called parapodia.
• This group contains organisms that lack clitellum and are dioecious.
• Eg., Nereis, Sallis

Oligochaeta

• They are mostly freshwater and terrestrial organisms.
• The body is segmented metamerically.
• Head, eyes, and tentacles are not distinct.
• They are hermaphrodites, but cross-fertilization takes place.
• Fertilisation is external.
• Cocoon formation occurs.
• Setae are segmented.
• They lack parapodia but do have clitellum. 
• This class of organisms is monoecious. 
• They have no free larval stage and develop entirely within cocoons. 
• For instance, Pheretima and Tubifex

Hirudinea

• Freshwater is where you’ll most likely find it. Some are marine, terrestrial, or parasitic.
• The body is segmented.
• The tentacles, parapodia, and setae are not present.
• The animals are monoecious.
• The body is dorsoventrally or cylindrically flattened.
• On the ventral side, they have an anterior and posterior sucker. 
• Cocoons are where the organisms lay their eggs. 
• During the development of the organism, there is no larval stage. 
• The anterior sucker’s mouth is ventrally located, while the posterior sucker’s anus is dorsally located.
• Fertilisation is internal.
• They are hermaphrodites.
• E.g., Hirudinaria

Archiannelida

• They are found only in the marine environment.
• The body is elongated without setae and parapodia.
• They are unisexual or hermaphrodite.
• Tentacles are present on the prostomium.
• Eg., Dinophilus, Protodrilus

Characteristics of Annelida

They are mostly found in the sea, either in saltwater or freshwater, but some are also found on land, burrowing or tubicolous, sedentary or free-living, commensal or parasitic. The body is triploblastic, bilaterally symmetrical, truly coelomate, and vermiform. The following are the characteristics of the organisms found in the Phylum Annelida:

1. The Annelids are coelomate and triploblastic.
2. They exhibit organ system-level organisation.
3. Their body is segmented.
4. They respire through their body surface.
5. Nephridia are the excretory organs.
6. They have a well-developed circulatory and digestive system.
7. Haemoglobin, which gives them a red colour, is found in their bodies. 
8. Annelids are well-known for their ability to regenerate.
9. Setae helps them in movement.
10. Most Annelids are hermaphrodites, which means they have both male and female organs in the same body. They reproduce sexually as well as asexually, and others procreate sexually.
11. E.g., Earthworms and leeches

Morphology

Annelids have bilateral symmetry, a worm-like appearance, and are repeated in each body segment. Metamerism ready access to expand their size and mobility by adding “compartments.” Identical myeloblast cells that give rise to identical mesodermal constructions during the embryonic stage are thought to be the source of this metamerism. The head, body, and pygidium are the three parts of the overall body (or tail). The clitellum is a reproductive structure that produces mucus to aid sperm transfer and gives rise to a cocoon within which fertilisation takes place; it appears as a fused tube band in the anterior third of the animal

Anatomy

An acellular, external cuticle protects the epidermis, but it is much thinner than the cuticle found on ecdysozoans and does not require periodic shedding for growth. Circular and longitudinal muscles are located on the inside of the epidermis. Every segment has setae/chaetae, chitinous hairlike extensions anchored in the epidermis and projecting from the cuticle. Annelids show the presence of a true coelom derived from embryonic mesoderm and protostome. Hence, they are the most advanced worms. Earthworms (oligochaetes) have a fully developed digestive system, including a mouth, muscular pharynx, oesophagus, crop, and gizzard. The gizzard connects to the intestine via an anal opening. A cross-sectional view of an earthworm (a terrestrial type of annelid) body segment is shown; each segment is defined by a membranous septum that divides the coelomic cavity into a series of compartments.

Conclusion

Members of the Phylum Annelida have segmented bodies, such as leeches and earthworms. Vermiform, segmented animals are included in the phylum Annelida. Internal anatomy also shows segmentation, which is known as metamerism. Annelids are protostomes. These animals’ neuronal and digestive systems are well developed. A clitellum is a specialised band of segments found in some species. Annelids have numerous chitinous projections known as chaetae, while polychaetes have parapodia. Suckers are found in the Hirudinea order. Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism. Internal segmentation is not present in the Hirudinea class.