Orientation/Symmetry

The earthworm is bilaterally symmetrical.

If you cut the earthworm down the center line, the left side of the body would be identical to the right side.

Anterior (cranal, towards the head

• the shorter region to one side of the clitellum is the anterior (head-end) of the animal
• usually more pointed than the posterior (tail-end) end of the animal

Posterior (caudal, towards the tail)

• longer region below the clitellum is the posterior (tail-end) of the earthworm  

Dorsal

• top-side of the earthworm
• sometimes darker than the ventral surface

Ventral 

• bottom-side of the earthworm
• lighter color than the dorsal surface

Periproct

• last segment of the earthworm

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  http://www.naturewatch.ca/english/wormwatch/resources/anatomy.html