Classification of the Phylum Annelida
The term annelida has been coined from either Greek word annulus meaning small rings of French word annular meaning arranged like small single rings. They are the first coelomate animals and as regarded as metamerically segmented animals.
Definition: The animal group who are triploblastic, coelomate and whose bodies are metamerically segmented and whose bodies are covered with soft cuticle is known as annelida.
The scheme of classification of the phylum annelida described here follows the same as appeared in the “Invertebrate Zoology”, Vol. I, written by Parker and Haswell and edited by Marshall and William (1974).
General characters of the phylum Annelida:
- Body is metamerically segmented, each segment is known as annulus, generally external segment correspond with internal segmentation, annuli are arranged in a linear sequence from anterior to posterior direction.
- Cuticle is soft and non chitinous.
- Body cavity is a true coelom, coelom is filled up with a fluid known as coelomic fluid. Coelom is divided by septum into many chambers.
- Body bilaterally symmentrical and triploblastic.
- In most annelids sea or cheaeta and parapodia are locomotory are locomotory organs, some move by muscles, i.e., seta or parapodia absent.
- Alimentary canal more or less straight, anus and mouth at opposite ends, digestion extracellular.
- Well developed blood vascular system, blood vascular system closed.
- Excretion by nephridia, each segment bears a pair of nephridium.
- Most of them respire by body wall, some possess gills.
- Well developed nervous system with ganglia and nerve cords. A pair of supra oesophageal ganglia, circum oesophageal connective and ventral nerve cord constitute the nervous system. Ventral nerve cord gives off ganglia in each segment.
- Most of them are hermaphrodite, some are unisexual, development direct or through larval stage, larva is known as trochophore larva.
Classification with reasons and examples:
Parker and Haswell classified phylum annelida into 4 classes as-
Ø Class-I : Polychaeta
Ø Class-II: Oligochaeta
Ø Class-III: Hirudinea
Ø Class- IV: Archiannelida
Class-I : Polychaeta
- Most of them are marine, 5-10 cm. in length and variously coloured.
- External segmentation and internal septum distinct.
- Each segment bears a lateral fleshy outgrowth help in swimming. These appendages are known as parapodia and parapodium bears many setae.
- Body is with a distinct head, head with eyes, pulp and tentacles.
- Clitellum is absent.
- Generally unisexual, fertilization external.
- Development through trochophore larva.
The Class Polychaeta has been divided in to two subclasses.
Sub Class I- Errantia
- Body is divided into many segments, except head and anal segment. All segments are equal.
- Parapodium is with an endoskeleton known as aciculum.
- Presence of mandible with teeth.
- Most of them are free swimmers, some may live in the tube.
Examples- Nereis, Aphrodite, Polynoe, Glycera, Syllus.
Sub Class I- Sedentaria
- Sedentary, tubicolous worms, living in calcareous or chitinous tubes secreted by epidermal glands.
- Head poorly developed with tentacles, pulps and feeding appendages.
- Trunk segments differentiated into or three regions due to dissimilarity in the forms of segments parapodia and setae.
- Pharynx not protrusible and devoid of jaws and teeth.
Examples- Chaetopterus, Arenicola, Amphitrite, Terebella, Sabella etc.
- Either freshwater or terrestrial forms.
- Head indistinct and without head appendages.
- Parapodia and bristles absent and setae remain embedded in the skin.
- Clitellum present.
- Sexes united, gonads few, permanent, development direct
Examples- Tubifex, Dero, Pheretima, Megascolex etc.
Class-III : Hirudinea
- Mostly aquatic forms, either fresh water or marine, but a few may be terrestrial.
- Body dorsoventrally flattened, divides into definite number of segments which may be divided externally into segments.
- Head distinct, without head appendages, may bear eyes.
- Parapodia, Setae, tentacles absent.
- Usually with two sucker, posterior one large and used for adhesion anterior one suctorial.
- Coelom filled with vacuolated parenchyma and botryoidal tissue.
- Blood vascular system haemocoelomic type.
- Fertilization internal, development direct, and are hermaphrodite.
Examples- Acanthobdella, Pontobdella, Hirudo.
Class-IV : Archiannelida
- Body simple, elongated vermiform and threadlike.
- Setae, parapodia, cirri absent.
- External segmentation indistinct, internal septum present.
- Hermaphrodite, gonads develop during reproductive season only.
Examples- Polygordius, Protodrilus etc.