Characteristics of Annelida - Classification upto order with characteristics and examples  

Triploblastic, metamerically segmented, tubular animals with coelom are classified as Annelida.

Metamerism and Metanephridium in Annelida Diagram 

The name annelida is derived from latin word annelus = ring + eidos = form, because the body of the members is made up of large number of segments which look like rings. About 22,000 species, divided into four main classes are included in this phylum. 

 Characteristics of Annelida

1. Habitat: Most of them are aquatic, some are terrestrial. Burrowing or tubicolous ( living in self constructed tube). Some are commensal and parasitic.
2. Organisation: organ system level of organisation.
3. Body: Elongated bilaterally symmetrical, triploblastic, coelomate and metamerically segmented into similar metameres. Tube within tube body plan.
4. Coelom:True coelom is present. They are the first group of animals to have a true schizocoelic coelom. Mostly well developed except in leeches. Usually divided into compartments by septa. Coelomic fluid with cells or corpuscles.The fluid act as hydrostatic skeleton,i.e high fluid pressure in the coelom maintain the shape of the body.
5. Bodywall: Internally lined by a thin peritoneum of squamous epithelium and externally by a single layer of columnar epithelium bearing microvilli. In between which, a mucopolysaccharide cuticle is deposited which is not made of chitin. Bodywall, dermo muscular. Outer muscle fibres circular, inner longitudinal.
6. Metameric segmentation: Segments called metameres or somites, except clitellum region.
7.  Locomotion: locomotion is by setae or by appendages called parapodia or by suckers.Setae are segmentally repeated chitinous bristles.
8. Digestive system: Digestive system is complete and digestion is extracellular by means of enzymes secreted by epithelial lining of the stomach (or intestine when the stomach is absent).
9.  Blood vascular system: First animals with closed circulatory system (refer to Circulatory System of Earthworm). Respiratory pigments are either hemoglobin or erythrocruorin dissolved in plasma called haemocoelomic system.
10. Respiration: Respiration by moist skin or gills of parapodia and head. Gaseous exchange takes place anywhere through the mucous- moistened body surface including parapodia and any other extensions of the body surface such as radioles and tentacles that can serve the function of gills.
11. Excretory system: Excretory organs of annelids are most typically paired metanephridia, but trochophore  larvae and a few families of polychaetes have protonephridia with flagellated solenocytes. The metanephridia that characterize most of the annelids usually occur in all or most body segments of oligochaetes ( e.g. Earthworm) and polychaetes, but in only the middle third of body of leeches. Although aquatic annelids excrete mainly ammonia, terrestrial species like earthworm synthesize urea as their major nitrogenous waste. (Refer: Excretory system of Earthworm)
12. Nervous system: It consists pair of dorsal cerebral ganglia (brain), nerve ring and double ventral nerve chord with nerves and ganglia in each segment.
13. Sensory organs include tactile organs, taste buds, statocysts, photoreceptor cells and sometimes eyes with lenses in some.
14. Reproduction: Hermaphrodite or sexes are separate. Cleavage spiral and determinate. Sexual demorphism. Fertilization external or internal indirect development or direct development.
15. Larva when present is a trochophore. Regeneration common.

Related Important Questions 

Question: What are distinctive characters of phylum annelida?

Classify the phylum  upto order mentioning five important characteristics  with example.

Answer:

Note: The distinctive characters of phylum annelida are mentioned above.

Classification of Annelida upto order:

The species of phylum annelida are divided into four main classes, mainly on the basis of presence or absence of parapodia, setae, metameres and other morphological features.

1. Class: Polychaeta 

(polys = many + chaite = hair ).

• Includes around 10,000 species mostly marine ( Some fresh water).
• Externally and internally segmented.
• Usually have trochophore larva.
• Lateral parapodia with numerous setae.
• Well developed heads bearing eyes, sensory tentacles, palps or radioles.
• They are mainly dioecious ( having male and female reproductive organs in separate individuals), with transitory gonads and epitokous reproductive forms            ( sexes seperate. Gonads temporary and in many segments). ( Epitokous reproductive forms:It is unique reproductive strategy for polychaetes or segmented worms where sexually immature worm gets transformed into a sexually matured worm.

Examples:Nereis, Aphrodite ( sea mouse), Chaetopterus, Serpula, Arenicola etc.

Class polychaete is futher divided into subclasses ( not order). They are:

Subclass: Errantia

1.  Errant (wandering) polychaetes. So they usually swim, crawl or tunnel activity.
2. Segments numerous and similar except for head and anal region.
3. Prostomium distinct with sensory structures well developed.
4. Trunk segment are similar and each is provided with a pair of well developed locomotory parapodia.
5. Extended pharynx, armed with Jaws or teeth, for getting food. Most are predacious.

Example: Aphrodite ( sea mouse). ( Family: Aphroditidae- with heavy oval bodies covered dorsally by long setae forming a mouse like fur).

* Scale worms ( family: Polynoidae and sigalionidae - with a dorsal surface covered by overlapping plates or elytra.)

* Nereids ( Family : Nereidae - large worms with proboscis bearing  one pair of chitinous jaws and a well developed head bearing four eyes and eight cirri.)

* Glycerids ( family: Glyceride -burrowing species with a very long proboscis armed with four jaws.)

Subclass: Sedentaria

1. Include polychaetes that live in burrows or tubes.
2. Body made of two or more regions with dissimilar segments and parapodia. 
3. No acicula and compound setae.
4. Prostomium small. Lack protrusible proboscis but often have long tentacles or prostomial radiolar crowns used in collecting particulate food.
5. Pharynx without jaws and teeth.

Example: Chaetopterus  (family: Parchment worms), Teribell, Arenicola, Sabella, Serpula.

2. Class: Oligochaeta 

(Oligo = few + chaite = hair)

•  Includes 3,500 species, mostly terrestrial and also some fresh water.Undergoing direct development. 
• Segmentation external and internal.
• Reduced head, lacking cephalic appendages, eyes, parapodia and have short and relatively few setae embedded in skin.
• They are mainly herbivorous, especially on decomposing vegetation.
• They are Hermaphrodite and fertilize their eggs externally, laying them into a chitinous cocoon secreted by glandular clitellum.
• Development direct, no larval stages.
• They lack gills, but have well developed vascular networks in the skin.

Examples: Tubiflex, Enchytraeus, Branchiobdella (parasitic), Lumbricus, Pheretima etc.

Taxonomic division of this class is mainly on the basis of technical differences in the reproductive system.

Oligochates are futher classified into Orders, as follows.

Order: Plesiopora plesiothecata.

1. Mostly aquatic.
2. Male gonopore in successive segment to the segment containing testes.
3. Spermatheca in the segment containing testes or nearby. 

Example : Dero, Tubiflex, Nais, Chaetogaster, Aelosoma.

Order: Plesiopora prosotheca

1. Spermatheca far anterior to segment containing testes.

Example : Enchytraeus.

Order: Prosopora

1. Mostly aquatic.
2. Male gonopore on the same segment or on the segment containing the second pair of testes.

Examples: Lumbaricus, Eisenia, Dendrobaena Pheretima, Megascolex, Allolobophora.

* Note: A super group Chaetopoda ( chaite = hair + pous = foot) including Polychaeta and oligochetae was created because of the fact that both the classes are provided with setae.

Class: Hirudinea

Hirudo= leech

• Includes some 500 species of leeches of which majority inhabit fresh water and a few are marine or terrestrial, ectoparasitic blood sucking or predatory.
• Body with fixed number of segments (33), ringed with annular grooves, dorsoventrally flattened. Segmentation external without internal septa. Parapodia and setae are absent.
• Both anterior and posterior ends have suckers. (Large posterior and often smaller anterior sucker, ventral to or surrounding the mouth.)
• Coelom invaded by connective tissue ( botryoidal tissue ) , muscle fibers and chloragogen tissue, is reduced to a mere systems of channels, the haemocoelomic sinuses.
• All leeches are hermaphrodite with only one male and one female gonopore. Fertilization internal. Clitellum is present and directly developing eggs (without larval stages) are layed in cocoons.

Examples: Acanthobdellida , Piscicola, Hirudo, Dinophilus Hirudinaria, Dina,  etc.

Division of class into order is mainly on the basis of feeding apparatus.

Order: Acanthobdella

1. Acanthobdella includes only one species which parasatizes salmon fishes.
2.  Considered primitive.
3. A pair of setae are born on segments 2,3 and 4
4. The first five metameres are internally separate.
5.  Coelom with compartments.

Order: Rhyncobdellida

1. Only aquatic ( includes fresh water and marine leeches.
2. Lacking jaws.
3. Have protrusible proboscis for sucking blood.
4.  Ectoparasitic.
5. Blood colourless and blood vascular system separate from coelomic sinuses.

Example: Glossiphonia, Placobdella, Helobdella, Piscicola, Pontobdella, Branchellion, Ozobranchus.

Order: Gnathobdell

1. Aquatic or terrestrial.
2. Ectoparasitic blood sucking leeches.Notably the madicinal leeches. e.g. Hirudo medicinalis.
3. Have no proboscis but usually possess three pairs of chitinous jaws.
4. Many are blood suckers but others are predatory.
5. Unlike other leeches, they have red blood.

Example: Hirudo, Hirudinaria, Haemadipsa.

Order: Pharyngobdellida

1. Include aquatic and semiterresterial leeches.
2. Some predaceous rather than parasitic.
3. Lack protrusible proboscis and jaws. 
4. May have one or two piercing stylets. 
5. Sucking in food by pumping action of the pharynx.

Example: Erpobdella, Dina.

Class: Archiannellida

1. About one dozen genera of small marine worms of unknown affinities.
2. Well segmented internally but lacks external segmentation. No parapodia and setae.
3. Has the nervous system embedded in the epidermis, which is often ciliated.
4. Often have solenocyte protonephridia.
5. Sexes usually separate.
6. Mostly trochophore larva.

Example: Polygordius, Dinophilus, Protodrilus.

Another important question related Annelids 

 

What is Metamerism? Types, Theories and Significances.