A closer look at segmented worms

Earthworms have a closed circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of “hearts” (aortic arches) which vary in number in the different taxa. A typical lumbricid will have 5 pairs of hearts; a total of 10. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms.

Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. (Typically, earthworms have four pairs of setae for each segment but some genera are perichaetine, having a large number of setae on each segment.) The whole burrowing process is aided by the secretion of slimy lubricating mucus. Worms can make gurgling noises underground when disturbed as a result of the worm moving through its lubricated tunnels as fast as it can.

Earthworm activity

Let’s say that a worm in its burrow wants to move forward. First, using its complex musculature, it makes itself long. Then it anchors the front of its body by sticking its front setae into the soil. Now it pulls its rear end forward, making itself short and thick. Once the rear end is in place, the front setae are withdrawn from the soil, but setae on the rear end are stuck out, anchoring the rear end. Now the front end is free to shoot forward in the burrow as the worm makes itself long and slender. Then the whole process is repeated.

There is a double nerve cord that travels the length of the body of the earthworm. Body movements are made because of this cord. The cords are swollen with cells controlling local activity and the brain integrates the sensory input.

Earthworms have no lungs. Their “breathing” consists of oxygen from the air passively diffusing through the skin into the body.

The earthworm has a body cavity through which the intestine passes with a front part that is modified to form special structures. The muscular throat sucks food into the food storage organ and it has a gizzard (a muscular organ in which food is ground into smaller pieces) that will grind food. They have an intestine (a region where chemical digestion and absorption occur). The intestine is tube-shaped and runs the entire length of the worm. It has a very tiny brain that is connected to a nerve cord that lies under the intestine. This comprises the nervous system of the earthworm.

The earthworm has a complete digestive system consisting of a tube with an opening at one end for taking in food, and an opening at the other for eliminating undigested residues. In between these two openings, food moves in one direction through regions specialized for processing and for transport. The earthworm does show discontinuous feeding patterns as its food may not be available at all times.

One important thing that earthworms do is to plow the soil by tunneling through it. Their tunnels provide the soil with passageways through which air and water can circulate, and that’s important because soil microorganisms and plant roots need air and water just like we do. Without some kind of plowing, soil becomes compacted, air and water can’t circulate in it, and plant roots can’t penetrate it.

Earthworms are very important to the soil as the worm digs through the soil by eating dirt. The soil will pass completely through the worm and out of the anus.

Earthworm droppings — called castings when deposited atop the ground — are rich in nitrogen, calcium, magnesium, and phosphorus, and these are all important nutrients for healthy, prospering ecosystems.

Nitrogen 5 times richer
Phosphorus 7 times richer Than the top 6” of soil
Potash 11 times richer

Biological. The earthworm is essential to composting; the process of converting dead organic matter into rich humus, a medium vital to the growth of healthy plants, and thus ensuring the continuance of the cycle of fertility. This is achieved by the worm’s actions of pulling down below any organic matter deposited on the soil surface (eg, leaf fall, manure, etc) either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 6″ of soil in which the worm is living.

Piet – Kapama River Lodge
06/06/2013

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