PHYLUM: ANNELIDA Salient Features and Classification

PHYLUM: ANNELIDA
Name of phylum annelida was first coined by Lamarck for the higher segmented u,ormms (Latin. annulus, liftle ring
Greek., eidos. form). App earance of metamerism represents their greatest advancement, so that they are called
segmented worms. They are the first animals tond have a closed vascular system,
• They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.
• They exhibit organ-system level of body organisation and bilateral symmetry.
• They are triploblastic, metamerically segmented and coelomate animals.
• Their body surface is distinctly marked out into segments or metameres (Latin, annulus : little ring) and,
hence, the phylum name Annelida.
• They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (sing. nephridium) help in osmoregulation and excretion.
• Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral
nerve cord.
• Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious.
• Reproduction is sexual.
Examples:
• Neries- Aquatic and commonalty called sand worm/Clamworm/Ragworm
• Polynoe- Bioruminescent and commonry cailed scare worm.Nereis,
• Pheretima (Earthworm)
• Hirudinaria (Blood sucking leech
(1) CLASS - POLYCHAETA
• Exclusively marine.
• Head distinct with eyes, olfactory palps and tentacles.
• Setae numerous, on lateral parapodia.
• Trochophore lava present.
Examples: Aphrodite (sea mouse), Eunice (Polalo worm), Chaetopterus(paddle worm), Arenicola (Lug worm),
Spinuculus (Pea-nut worm), Sabella (Peacock worm), Serpula (Fan worm), polynoe(Scale worm,
Bioluminiscent) Nereis (Clamworm orSandworm orNeanthes ), Odontoltis(Fire worm).
(2) CLASS - OLEIGOCHAETA:
Mostly, terrestrial, some in fresh water.
Glandular clitellum present for cocoon formation.
Hermaphrodite. Testes anterior to ovaries.
• Pheretima (North lndian earthworm)
• Lumbricus: European andAmerican earthworm
• Megascolex: South lndian earthworm
• Chaectogaster orAelosoma : Smallest annelid 1 mm length.
• Megascloides australis : Largest annelid 2.5 metre length.
• Tubifix: Blood worm
• Branchiobdetla(Parasitic)
(3) CLASS - HIRUDINEA:
Freshwater, marine or terrestrial, generally ectoparasite, blood sucking (sanguivorous) or
carnivorous.
Body with fixed number of segments (33).
Each segment subdivided externally into annuli.
Segmentation external without internal septa.
parapodia and setae absent.
Both anterior and posterior ends of body with suckers.
Hermaphrodite, numerous testes and one pair of ovaries with one male. and one female gonopore.
Fertilization internal. Development in cocoons, directwithout larval stages.
Examples : Glossiphonia (flattened leech), Acanthobdella (Russian leech), piscicda (fish leech), pontobdella(skate
sucker' marine leech), Hirudo medicinalis (medicinal leech), Hirudinaria granulosa (catge leech)Aulostoma (horse
leech), Haeniodipsa (land leech).
(4) CLASS - ARCHEANNELIDA :
lt is small, marine worm.
Segmentation internal. No parapodia and setae.
Sexes usually separate. Gonads numerous, fertilization internal. Sually trochophore larva . Example :
Polygordius; Dinophilus, protodritus.