Earthworms are invertebrates – they lack a rigid skeleton. Their skin is permeable, allowing water to pass through. They must live in damp habitats or they will dry out and die.
The body cavity (coelom) between the muscular body wall and intestine is divided into segments, with a membrane (septum) between each.
Each segment is like a sack, and contains various organs, depending on its position in the body. There are two sets of muscles in each segment: a ring of circular muscles that can constrict or narrow the segment, and longitudinal muscles that can shorten the body. Using these stretching and contracting muscles, they push themselves forward.
The fluid in each segment cannot be compressed, and contraction of the muscles causes the segments, and thus the body, to change shape. Swelling enables the body to open up gaps in the soil, while a limp body can move through tight spaces.
The number of segments in mature earthworms ranges from about 50 to over 500.
New worms from old
Most earthworms have amazing powers of repairing damage to their body. But is it true that if you cut a worm in half, you will end up with two worms? In fact, only one part may survive, and even then, regenerating the missing part depends on where it has been cut.
On all but their first segment, earthworms have hairs, or chaetae, which feel like bristles and aid movement by gripping surfaces. The number on each segment can be important in identifying different types. Earthworms in one New Zealand genus (Hoplochaetina) have at least 12 individual chaetae per segment (H. polycystis has about 70), while the lumbricids (which are non-native) always have eight per segment.
Many body organs are repeated in adjacent segments – for example nephridia, which function like kidneys, occur on most segments. Often there are paired hearts in two to five adjacent segments, and paired testes in two adjacent segments.
The worm’s anterior (front section) has the segments with the most specialised organs. The pharynx, behind the mouth, and the grinding gizzard together occupy about 10 segments. There is a thin oesophagus (the tube linking the throat to the stomach), and then the simple intestine begins at about the 15th to 20th segment, running to the terminal anus.
Earthworms are hermaphrodite: each individual has both male and female organs. But they are not self-fertile, and structures for copulation lie on the rear part of the body. The positions and degree of development of the clitellum (a glandular swelling that produces a cocoon for the eggs), and various pores and papillae (bumps), are very important in identifying species.