THE EARTHWORM DISSECTION

For ease in identifying organs or corresponding parts in the earthworm, numbers will be used.  These will refer to segments
counting from the anterior end.

A. EXTERNAL ANATOMY
Note again the uniform segmentation of the worm and the lack of conspicuous appendages. The number of segments usually exceed 100 in number.

The dorsal surface is pigmented and marked with a dark line, the dorsal blood vessel.  The ventral surface is pale and
somewhat flattened.

A swollen zone (32-37) near the anterior end is the clitellum, a glandular area which secretes a cocoon in which the
young earthworms develop.

The mouth opening, overhung by a small prostomium (again, not a true segment) devoid of visible sense organs, is in the
ventral region of the first segment, the peristomium.  An anus is in the center of the last segment.

By running the tip of the finger forward over the ventral surface of the body, thin bristles, chaetae, may be felt.  Thes  are moved by internal muscles and assist in locomotion.  Careful examination will reveal these to occur in pairs, four pairs to
each segment; two pairs are ventral, the other two pairs are nearly lateral in position.

Two sperm grooves run between the inner and outer rows of chaetae from the clitellum to the sperm duct openings.

Other openings to the surface should now be identified.

1.   Sperm duct opening (15) are conspicuous ventral slits on the ventral surface.
2.   Oviduct opening (14) are small apertures just lateral to the ventral row of chaetae.
3.   Seminal receptacle openings (in grooves between 9 and 10, and 10 and 11) are small slit-like apertures in line with the lateral row of chaetae.  Low power magnification will assist you in seeing these and the next two types of openings.
4.   Nephridiopores are external openings of the excretory organs, the nephridia; the pores, not present o the first and last few segments, like just anterior and lateral to the ventral row of chaetae.
5.   Dorsal pores are openings from the coelom to the outside. These lie in the dorsal line between segments.