Table : Important examples of Phylum Annelida
Scientific
nameCommon name Important featuresClass-PolychaetaNereis Sand worm or ragworm or
clamworm
• Nereis is a cosmopolitan marine polychaete, living in U-shaped burrows in
sand or mud.
• It is carnivorous and nocturnal in habit.
• Body is divisible into head, trunk and pygidium.
• Peristomium is the first segment of the body which is devoid of parapodia
but bears two pairs of thread like peristomial cirri on each side.
• Pygidium is the last segment of body and is also devoid of parapodia and
setae.
• Parapodia are flattened, fleshy, vertical flap-like outgrowths of body wall on
the lateral sides of trunk segments. Each parapodium bears a bundle of long,
fine, stiff, chitinous bristles, the setae or chaetae. Parapodia serve the dual
purpose of locomotion and respiration.
• Body wall consists of cuticle, epidermis, musculature and coelomic
epithelium or peritoneum.
• Peritoneal cells lining the coelom secrete the coelomic fluid and give rise to
gonads during breeding season.
• Gills or any other special organs of respiration are lacking. Respiration is
carried out by whole body surface, more specially by thin flattened lobes of
parapodia.
• Nitrogenous waste is mostly ammonia.
• Sense organs include prostomial tentacles, prostomial palps, nuchal organs,
peristomial cirri and 2 pairs of eyes.
• Most of the species of Nereis are unisexual (dioecious). Gonads (testes and
ovaries) are neither distinct nor permanent organs and formed only during
breeding season.
• In Nereis there are no gonoducts.
• At sexual maturity, most of the posterior segments, filled with gametes, exhibit
morphological and anatomical differentiation. These constitute the sexual
region or epitoke of worm. Few anterior segments which do not take part in
gamete formation, constitute the asexual region or atoke.
• Sexually mature worm with these two regions is known as Heteronereis and
the phenomenon involving transformation of non-sexual individual into sexual
individual is referred to as epitoky.
• Fertilisation is mostly external and takes place in sea water.
• Development is indirect including larval stage, i.e., trochophore.