Segmented worms: Phylum Annelida

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.

Annelid features:

• 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. This makes annelids different from nematodes, which have a pseudocoelom.
• 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).
• Specialized excretory organs: metanephridia. Most animal phyla have some sort of specialized excretory organ, but the sponges and cnidarians do not.

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.

The photo above shows a cross section from near the middle of the earthworm. The gut occupies much of the middle of the picture. Rather than just being a simple tube (as in nematodes), the earthworm gut has a typhlosole, a structure hanging from the top of the gut that provides more surface area for secretion and absorption. Much of the rest of the cross section is occupied by the large coelom.

• Circulatory system: 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.
• Nervous system: The earthworm has a ventral nerve cord, which is supplied with blood by the subneural blood vessel.

At higher magnification, you can see more of the detailed structure.

• Intestine: The intestine has an inner epithelial layer that is derived from embryonic endoderm. Lining the outside of this epithelial layer is another layer of tissue, which is derived from embryonic mesoderm. (This is different from a nematode's intestine, which has only the inner epithelial layer.) In some of the earthworm slides, there may appear to be three intestines, because there are small ceca that branch off the main intestine.
• Coelom: Since the gut is completely surrounded by mesodermal tissue, the mesodermal tissue forms a complete lining of the coelom. That's why this is classified as a true coelom.
• Muscles: 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. Each segment has a coelomic cavity filled with fluid, which can act as a hydrostatic skeleton (this concept will be covered in lecture). 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. Compare the earthworm's musculature to the much simpler nematode, which only has longitudinal muscles.
• Metanephridium: The metanephridia are osmoregulatory and excretory 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. Each segment has a pair of metanephridia.

Earthworms are hermaphrodites — they are both male and female at the same time. In the picture above, from the anterior end of a worm, you can see the seminal vesicles. These structures store sperm produced by this individual and aid in the maturation of the sperm.

References & further reading

Campbell Biology, Chapter 33. See figure 33.25 (10th edition) for a diagram of earthworm anatomy.

Earthworm on Wikipedia.

Lumbricus terrestris on Invertebrate Anatomy Online, by Richard Fox, Lander University.