Phylum Annelida (5th Phylum)

• Free-living, terrestrial or aquatic form (freshwater or marine).

• Body is long, cylindrical and metamerically segmented.

• Body wall consists of cuticle, epidermis and musculature.

• Body cavity is a true coelom, lined by epithelium.

• Digestive system is complete.

• Respiration is either through skin or through gills.

• Excretion with the help of characteristic nephridia.

• Presence of a circulatory system (closed type). Hemoglobin dissolved directly in the plasma.

• Nervous system consists of a nerve ring and a double ventral nerve cord with segmental ganglia.

• Locomotion with the help of setae or parapodia.

• Reproduction by sexual methods. Either unisexual or bisexual development may be direct or indirect.

• Body has a true coelom.

           fig. 10.13 - Examples of Annelida

The phylum is divided into four classes

Characteristics	Class Polychaeta	Class Oligochaeta	Class Hirudinea	Class Archi annelida
Habit and habitat	Free living marine	Free living forms found in most soil	Fresh water forms Temporary ectoparasites	Marine forms
Locomotor Structure	Parapodia	Setae	Absent	Absent
Lifecyle	Unisexual with indirect development	Bisexual with direct development	Bisexual with direct development	Bisexual with indirect development
Examples	Nereis (reg worm)	Pheretima, Megasolex (earth worms)	Hirudinaria (leech)	Polygrdius

 

fig. 10.14 - Anterior Segments of Nereis

                fig. 10.15 - Earthworm Anatomy