Place the earthworm on the dissecting tray, dorsal
side up, and pin into position (Figure 2A). To expose the internal organs,
dissect the worm as outlined in Figure 2B-F.
A. Pin through prostomium. Pin through last (anal)
B.With forceps, lift dorsal skin. Cut slit at base
of forceps (a) insert scissors and cut a line, slightly off center, through
to the anus (b). Caution: Be careful to cut only as deep as the skin to
avoid damaging internal organs.
C. Beginning at the anal end, hold body wall with
forceps and with razor or scalpel, cut through septa on both sides of the
intestine. Cut to within 1/2 to 1 inch of the clitellum.
D. Pin the body wall to the dissecting tray as
E. Cut through ciitellum anteriorly. Sever the
septa and pin body wall to tray as illustrated in F.
Figure 3 shows the general location and structure of the
digestive system. The other organ systems have been omitted. Locate:
a. The mouth is at the anterior end. The opening is located
just below the overlapping prostomium.
b. The mouth leads to a slightly expanded and muscular
pharynx which is usually covered by three pairs of whitish seminal vesicles.
These will be examined later. Food taken in by the animal is passed on
by muscular contractions in the pharynx through the esophagus to
the crop where it is temporarily stored,
c. The crop opens into a thick-walled, highly muscular
where food, with the aid of small soil particles taken in during feeding,
is ground up. It is then passed into the intestine where digestion
and absorption occur.
d. Solid waste products of digestion are passed to the exterior
through the anus.
An interesting feature of the circulatory system is that
it is a "closed" system in which the blood circulates within a series of
blood vessels. The blood is red because it contains hemoglobin (the pigment
that gives the red color to human blood).
The major vessels of the circulatory system are a dorsal
longitudinal vessel lying on top of the digestive tract and a ventral
blood vessel lying below it. These two vessels are connected with each
other by a number of vessels passing around the digestive tract. Five pairs
of these (located in segments 7 to 11 inclusive) are larger than the others
and comprise the "hearts." Pulsation of the hearts causes circulation of
Most of the other blood vessels are difficult to observe
unless the material has been sectioned for microscopic study.
To obtain a clear view of the reproductive organs, cut
through the intestine near the clitellum. Carefully lift it and ease it
free as far forward as the posterior end of the pharynx. Then cut it out.
Although earthworms are monoecious (They possess a complete
set of male and female reproductive organs.) they nevertheless undergo
cross-fertilization during copulation. Two worms come together along their
ventral sides and become temporarily joined together by the secretion of
a "slime tube." During copulation sperm cells are reIeased from the
vesicles and stored in the seminal receptacles of the opposite
earthworms The animals separate and the clitellum then secretes a mucus
ring. This ring slides over the anterior segments and picks up eggs from
the oviducts in the 14th segment and sperm as it slides over the anterior
end of the worm to form the egg cocoon, from which the young eventually
hatch. The seminal vesicles have previously been described as lying alongside
of the crop and gizzard. The testes are inside of these bodies and cannot
be seen. Near the seminal vesicles, in segments 9 and 10, are the four
small, spherical seminal receptables. By careful examination with a hand
lens or dissecting microscope you may be able to locate the single pair
of ovaries in segment 13.
The nervous system in the worm is difficult to study.
Its major component is the ventral nerve cord which runs the length
of the worm on the inner ventral surface (Figure 3). At its anterior end
the cord divides and passes around the front part of the pharynx where
it enlarges to form two swellings - the cerebral ganglia, These
might be considered primitive "brains."
Along the length of the cord, lateral nerves are given
off which go to the muscles of the body wall.
Each segment of the body, except the first three or four
and the last one, contains a pair of excretory structures called metanephridia.
These coiled tubular structures, lying next to the body walI, open to the
exterior by a pore called a nephridiopore internally they are connected
to the septum of the segment just anterior to them. Each nephridium collects
fluid wastes from the segment anterior to the segment in which it is located.