Anatomy

Earthworms are invertebrates, which means they have no back bone (unlike snakes, which do have a back bone). Earthworms are annelids and are members of the class Oligochaeta, because they crawl using circular and longitudinal muscles which are located under the epidermis. Each segment also has bristle like setae (see figure 1) which help to anchor their segments as they crawl. Unlike humans, earthworms do not have a well developed respiratory system. Instead of lungs, they breathe through their skin which needs to stay moist for breathing.

Earthworms are annelids, which means their bodies are segmented. Earthworms have bilateral symmetry, which means if you cut an earthworm down centre, you would find that the left and right sides the its body are identical (mirror images of each other).

Unlike many other invertebrates, the circulatory system is fully closed. One large blood vessel runs the length of the body, immediately beside the gut. Two to five pairs of muscular blood vessels extend from the central vessel and function as hearts to drive the circulatory system.

The first segment is specially named the peristomium (see figure 1) and contains the mouth. There is a small tongue-like lobe just above the mouth called the prostomium (see figure 1). The prostomium is used by the earthworm to “see” its environment - as earthworms have no eyes, ears, nose or hands, it depends on the prostomium and skin to help it feel its way through the soil. As the earthworm tunnels through the soil, it excretes mucus from its body. This mucus reacts with the soil of the tunnel walls and forms a type of cement which makes the tunnel walls stable so that the tunnel can be reused. The earthworm brain is actually a fused pair of nerve ganglia, mostly located in the third segment. There are three giant nerve fibres that run the length of the body, around the gut. These fibres transmit impulses from the brain which control rapid body movements.

About one third of the way down the earthworm (from the head) is the clitellum (see figure 1). The clitellum is a swelling of the skin and can only be seen in earthworms that are ready to reproduce. It may be white, orange-red or reddish-brown in colour. Earthworms are ready to mate when their clitellum is orange. Most of the material secreted to form earthworm cocoons is produced within the clitellum. The number of the segments to where the clitellum begins and the number of segments that make up the clitellum are important for identifying earthworms.

The very last segment is called the periproct (see figure 1) and contains the anus.

Except for the first and last segment all the other segments have eight setae located around each segment. The setae look like small bristles sticking out of the earthworm’s skin. The setae can be retracted and are for moving through the soil. The bristle-like setae anchor the segments as they crawl.

How to identify different earthworms. The number of segments from the peristomium to the clitellum and the number of segments which make up the clitellum are species specific in earthworms. This means that if two earthworms have different numbers of segments to the start of the clitellum, they are different earthworm species . In Canada, there are three families of earthworms represented: Lumbricidae, Acanthodrilidae and the Sparganophilidae. There are some species of earthworms that are native to North America and Canada: Aporrectodea bowcrowensis, Bimastos lawrenceae, Arctiostrotus perrieri, Arctiostrotus vancouverensis, Toutellus oregonensis and Sparganophilus eiseni. Currently, 25 different earthworm species have been found and identified in Canada. Perhaps with your help we can find more!

Figure 2. Earthworm cocoon.

How to tell juveniles from adults. Those earthworms without genital markings such as the clitellum, tubercula pubertatis (see figure 1), or genital tumescence are juveniles. This stage of the life cycle is located between the hatchling phase and the appearance of genital markings (adult stage).

Figure 3. Aestivating earthworm.

Other stages in the life of an earthworm. Earthworms reproduce by laying a cocoon, a sac that contains the earthworm’s eggs. The cocoon is formed at the clitellum, and then travels from the clitellum to the head. Here it slides off the earthworm’s body and is deposited into the soil.

Earthworms can enter into periods of inactivity, or dormancy, as the result of unfavourable conditions (e.g. dry periods). This is known as aestivation. During aestivation, the earthworm curls up into a knot and becomes quite pink.