Wednesday 20 June 2012

NEREIS-BODY DIVISION

BODY DIVISION-The body of nereis is metamerically divided into a number of metameres or segments arranged in a linear series.The number of segments is fairly constant for a species about 80 in N.cultrifera and N.dumerilli and about 200 in N.virens.The distinct regions can be identified in the body .
HEAD-Nereis possess well developed head corresponding to its active life and predaceous habit.It lies at the anterior and consists of two parts.
1> PERISTOMIUM
2>PROSTOMIUM
PERISTOMIUM-It is large,segment like part of the head surroundings the anteroventrally situated and transversely elongated,slit like mouth.It is formed by the fusion of first two embryonic segments during cephalization.It differ from a trunk segments in being longer, in lacking parapodia and in the presence of two  pairs of thread like peristomial cirri on each side.These are homologous with the notopodial cirri and neuropodial cirri of the parapodia of trunk segments.The dorsal cirri are longer than the ventral ones.
PROSTOMIUM-It is a roughly triangular ,dorso-ventrally flattened fleshy lobe lying above and in front of the mouth.It bears two pairs of simple pigmented eyes on the dorsal surface a pair of short cylindrical sensory prostomial tentacles.The palps are contractile and their small distal joints can be retracted into their large proximal joints.

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