Introducing the Worm
Let us introduce the Worm Charles Darwin was fascinated by worms. In his book "The Formation of Vegetable Mould by the Action of Worms" (1896), Darwin wrote "Worms have played a more important part in the history of the world than most persons would at first suppose." |
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Earthworm Biology
Anatomy
Earthworms, as everyone knows, are usually red or pink, long, cylindrical, and divided into rings or segments. Closer inspection shows that adult specimens have a clitellum, often called a saddle because it is a thickened area, usually paler on the upper and side surfaces of the body.
Earthworms have no special breathing organs - oxygen is absorbed and waste gases are eliminated by the skin by a network of fine blood vessels just under the skin surface.
Gland cells secrete mucus to moisten the body surface to allow this process to occur. The mucus also lubricates the body to enable it to move through the soil.
The body structure of an earthworm is basically a rather simple arrangement of one tube inside another. The outer tube is the muscular body wall, and the inner tube the digestive system, with a fluid filled space separating them.
Digestion
The digestive system is essentially an unbranched tube running from the mouth at the extreme front to the anus at the other end of the body. Just inside the mouth the tube expands into a strongly muscular pharynx. The pharynx turns inside out so that it protrudes through the mouth to surround and grasp pieces of food. The food is then pulled back into the mouth and passes back into the worms' gizzard. The gizzard grinds the food before it passes into the worms' intestines and eventually through to the anus.
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Left: Scanning electron Microscope photograph of an earthworm showing details of segment and setae. Magnification x 11.
Right: Close-up of earthworm setae. Magnification x 125.
How They Move
Movement involves a quite complex set of co-ordinated actions and reactions involving the earthworms' muscles, small bristles on the body wall (called setae), and the fluid filled body cavity. It can be simplified by saying that a worm bunches up like a spring and uses the bristles at the back like an anchor, to push itself forward.
Brain, Heart.
An earthworm has a brain, a small bundle of nerve tissue located on top of and right a the frong of the pharynx. It is connected to a nerve cord that runs along the body cavity, under the gut, providing nerves for muscles and other organs in each segment of it's body.
Blood is pumped throughout a worms' body by several (usually 3, 4 or 5) pairs of hearts, which are prominent vessels surrounding the oesophagus, one pair in each of several successive segments.
Breeding
Earthworms have both male and female reproductive organs in the one individual. There are two pairs of testes (male organs) and one pair of ovaries (female organs) in most species.
During mating, two worms align themselves in a head to tail position, touching each other over a length of around 35 front segments. A thick layer of mucus is produced and the two worms discharge sperm cells, and then separate from each other.
A broad ring forms around each worms' body which receive the eggs and sperm cells as the worm works its body backwards from inside this ring. Eventually, thr ring passes over the worms' head to form a cocoon, which houses a number ofeggs. Baby worms emerge from the cocoon after a period of approximately three weeks.
From: Earthworms for Gardeners and Fisherman. CSIRO 1978. East Melbourne.
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