Polychaeta

Characteristics :

(i) Body bears distinct head with sense organs 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 sandworm, Chaetopterus-the peddle worm, Aphrodite-the sea mouse.

Nereis (Sandworm or Clamworm)

Cosmopolitan, marine polychaete, usually found on sandy shores between tide marks. It lives mostly in U-shaped burrows.

The burrow is coated with mucus which binds together fine particles of sand. The worm is gregarious, nocturnal, carnivorous with separate sexes.

The animal can swim in water by lashing body movements. The body is divided into three parts: (a) head, (b) trunk & (c) Pygidium.

Head consists of prostomium & peristomium. The prostomium bears a pair of small tactile tentacles & a pair of stout palps.

On the dorsal surface of the prostomium, there are present two pairs of black eyes. The prostomium bears four pairs of long cirri. On ventral side of prostomium, mouth is present.

Trunk consists of 80-200 similar segments, each characterized by the presence of a parapodium on each lateral side.

A parapodium possesses a number of setae for fixation and protection. Parapodia serve the dual purpose of locomotion and respiration. Pygidium has anus and two long anal cirri.

Nereis is unisexual and its reproductive phase is called Heteronereis.

The posterior segments of sexually mature worm are filled with gametes. These regions exhibit morphological and anatomical differentiation.

This region is known as epitoke whereas the anterior segments which do not take part in gamete formation is known as atoke. Fertilization is external. Development is indirect. The larva is trochophore.

Further Reading : 

1. Gymnosperms

2. Fungi

3. Angiosperms