Earthworms exist in almost every part of the world, except the arid and polar regions. They are invertebrates, and their body color can be pink, brown, red, and reddish-violet. You may find blue-colored earthworms in Philippines and some green-colored species in England. The body length of an earthworm may vary according to the species, and can range between one centimeter to four meters. This organism has a tube-shaped, segmented body with a moist and slimy outer covering. The ring-like segments are more specialized in the anterior part of its body.
The opening at the anterior end of an earthworm is the mouth, and in some species, it is covered by a lobe called the prostomium, which is like a sensory receptor. Earthworms do not have nose, eyes and ears. Hence, the prostomium along with the sensory receptors on their skin, help these organisms to collect information about the environment.
On the ventral surface of the body, there are claw-like structures called setae, which are mainly used for locomotion. There is an armband-like swollen part called clitellum, which is located towards the anterior end of the body. This part secretes mucus while mating, and produces the egg case, inside which fertilization occurs. It is more prominently seen in mature worms, and are lighter in color than the other parts of the body. The anus is located in the last segment.
Earthworms have a very simple circulatory system with two blood vessels, that run through the entire length of their body. The ventral blood vessel carries blood to the rear part of the body, and the dorsal blood vessel supplies to the anterior parts, where the series of hearts (aortic arches) is situated. The number of hearts may vary for different species, but a typical earthworm has five hearts. These hearts pump blood to the ventral vessel, from where it is redirected to the capillaries on the body wall and other organs.
Digestive and Excretory System
The digestive system of an earthworm is like a tube. It has no jaws or teeth, and the soil is sucked into the mouth with the help of the muscular pharynx. From there, the soil passes through the tube-like esophagus, which ends in a round organ called the crop. The crop serves the purpose of storing food. From the crop, soil is forced to the gizzard, which is a strong muscular organ. It expands and contracts, grinding the soil and the sand particles together. The perfectly ground up soil is passed to the intestine, which starts from segment 19 and ends with the anus at the end of the body.
Here, the enzymes break down the organic matter in the soil, and this digested food is absorbed by the blood circulating through the intestine walls. The leftover is excreted through the anus, and is found in the form of castings. Some water is also absorbed by the blood during the digestion process. The remaining water is eliminated through the pair of pores called nephridia, present in every segment (except the first three and the last one).
An earthworm has both male and female reproductive organs, but are not able to fertilize themselves. They cross fertilize by attaching their clitellum and exchanging sperms. The sperms are produced and stored in the seminal vesicles (2 or 4 pairs in segments 9 to 12), where the testes are located. A pair of ovaries are located in segment 13, which produces eggs to be released through female pores on segment 14.
During copulation, earthworms use the claw-like bristled structure in their body called setae, to hold on to each other, with the heads facing opposite directions. They exchange sperms and then separate from each other. These sperms are stored in internal sacs called spermathecae, located in any segment from 9 to 12. After some time, the clitellum produces a substance, with which a ring is made around the worm. The worm wriggles out of the ring, and during this process, injects eggs and sperms into the cocoon.
The ring of the cocoon is released through the anterior portion of the earthworm, and the spermathecae is located between the head and the clitellum. Once released, the ends of the cocoon are sealed, and embryos are developed in this incubator-like structure.
There is no specific respiratory system in earthworms. They absorb oxygen through their moist skin and emit carbon dioxide. Their nervous system is also very simple, with a brain and a nerve cord which runs through the length of the body. The brain has the function of directing the body movement with respect to light. There is no skeleton in earthworms.
The earthworm may appear as a tiny creature, which has a long, tube-like body with tapering ends. In fact, they are highly evolved worms, helpful to mankind in many ways.