Annelida

Annelida

The body is elongated, cylindrical or flattened. It is divided into segments by ring-like grooves, the annuli. The segmented worms are triploblastic, i.e., they develop from three germ layers. The body-cavity is a true coelom. As it is lined by a mesodermal epithelium. It is divided by vertical septa into compartments. A closed circulatory system appeas for first time in animal kingdom. The excretory system consists of coiled tubes, the nephridia.

Classification: the phylum Annelida is divided into four classes; Polychaeta, Oligochaeta, Hirudinea and Archinnelida.

Polychaeta

(i) Body bears a distinct head with sense organ and unjointed locomotory appendages called parapodia.

(ii) Setae are long, numerous and occur in the Parapodia.

(iii) Clitellum is absent.

(iv) Sexes are separate.

(v) Development includes a larval stage.

Examples Nereis- the sand worm, Chaetopterus- the paddle worm, Aphrodite- the sea mouse.

Oligochaeta

(i) Body lacks head and parapodia

(ii) Setae are small and fewer

(iii) Sexes are united

(iv) Development is direct

Examples Pheritima –the earthworm, tubifex- the blood worm

Hirudinea

(i) Body lacks head and parapodia

(ii) Setae are also absent

(iii) Clitellum is developed during the breeding season

(iv) There is an anterior sucker and often also a posterior sucker for locomotion and feeding

(v) Development is direct

(vi) Sexes are united

Examples: Hirudinaria – the cattle leech. Pontobdella-the fish leech

Archinnelida

(i) Head with sense organ is present

(ii) Parapodia and setae are usually absent

(iii) External segmentation is faint, internal segmentation by septa

(iv)

(v) Sexes may be separate or united

(vi) Development includes a larval form.

Examples: Polygordius