Animals > Segmented worms
Segmented worms: Phylum Annelida
Reading: Campbell, Chapter 33
Annelid worms are quite different from flatworms or nematodes. Having a segmented body with a true coelom makes them considerably more complex in their structure and their movements.
- Three tissue layers in embryo. Almost all animals share this basic feature; the sponges and cnidarians are exceptions.
- Segmented body. Contrast this with nematodes, which have unsegmented bodies.
- True coelom: The coelom of an annelid is a large space in which the internal organs form. Since the body is segmented, the coelom is segmented, too.
- Complete digestive tract: The digestive tract run througout the length of the body, and different regions show a significant degree of specialization (unlike flatworms or nematodes).
- Closed circulatory system: Blood is contained within the circulatory system. This is particularly important given that the segmented body prevents coelomic fluid from cirulating througout the body (as it does in nematodes).
Bloodworms (class Polychaeta)
Bloodworms such as Glycera are marine worms; they wriggle around in the mud and eat small pieces of organic material.
The body plan of annelids, with separate segments surrounded by layers of muscle, makes these worms good at burrowing into a substrate, whether on land or under the sea.
Bloodworms are often used as fishing bait; it's possible to find them
by digging in the ocean mud at low tide in certain locations.
Earthworms (class Oligochaeta)
Earthworms are simultaneous hermaphrodites -- each individual produces both eggs and sperm at the same time. When a pair of earthworms mates, each individual gives sperm to the other. Sperm is released through a male genital pore and received by the female genital pore on the other individual.
After mating, the two individuals separate. Each secretes a thick mucus ring from its clitellum. The worm then secretes its own eggs, along with the sperm from the other individual, into this mucus layer. The worm moves away and leaves the mucus ring behind. The outer layer of mucus hardens and shrinks, forming a protective cocoon in which the eggs get fertilized and larval development occurs.
Earthworm cross sections
We have several slides with cross sections of earthworms. Before examining the slides, study the dissected specimen of a whole earthworm. You'll see that different regions of the body may contain different organs; keep that in mind as you look at the cross sections.
Near the anterior end of the animal, the largest organs in the coelom are the seminal vesicles. These structures store sperm produced by this individual and aid in the maturation of the sperm.
Like all annelids, the earthworm has a closed circulatory system. In the cross sections, you can see the blood contained in the dorsal and ventral blood vessels, along with a couple of others.
The earthworm has a ventral nerve cord, which is supplied with blood by the subneural blood vessel.
The cross section shown below is from the middle of the animal. Note how large and empty the coelom is.
Here is a higher-magnification view of a similar cross section:
The metanephridia are osmoregulatory and excretor organs. They form narrow tubes that curl around the coelom and eventually empty through the body wall to the outside. Cross sections such as this one only show part of a metanephridium.
The coelom is completely lined with tissues that are derived from embryonic mesoderm; in the lab on tissues and development, you'll be introduced to embryonic tissue layers and their significance. The intestine is lined with intestinal epithelium derived from endoderm on the side facing the intestinal lumen; the side facing the coelom is covered with another tissue layer, derived from mesoderm.
In segmented worms, the muscles are also segmented. Longitudinal muscles run along an anterior-posterior axis; when they contract, they tend to shorten the segment. Circular muscles form a ring around each segment; when they contract, they tend to force the segment to become smaller in diameter and thus longer. Using longitudinal and circular muscles, earthworms can force their bodies through the soil by making one part of the body long and skinny while another becomes short and thick.
Another cross section, further toward the posterior end, shows more or less the same features described above.
This page updated September 19, 2011