Anatomy of an Earthworm

Earthworms exist in almost every part of the world except the arid and polar regions. They are invertebrates (without bones) and their body color can be pink, brown, red or red-violet. You can find blue-colored earthworms in Philippines and some green color species in England. Their body length may vary according to the species and can range between one centimeter to four meters. Earthworms have a tube-shaped segmented body with a moist and slimy outer part. The ring-like segments are more specialized in the anterior part of their body.

External Features

The opening at the anterior end of an earthworm is the mouth, which in some species is covered by a lobe called the prostomium. It is like a sensory receptor. They do not have nose, eyes and ears. Hence, the prostomium along with the sensory receptors in their skin help the earthworms in collecting 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 (near to the head), which is called the clitellum. This part secretes mucus while mating and produces the egg case, where 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.

Circulatory System

Earthworms have a very simple circulatory system with two blood vessels, that run through the length of their body. The ventral blood vessel carries blood to the rear part of the body, and the dorsal blood vessel to the anterior, where the series of heart (aortic arches) is situated. The number of hearts may vary for different species, but a typical earthworm possesses 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. They have no jaws or teeth and the soil is sucked into the mouth with the help of muscular pharynx. From there, the soil passes through the tube-like esophagus, which ends in a round organ called 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 (loretto) at the end of the body. Here, 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).

Reproductive System

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 9-12), 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 worms produce cocoons (egg case) from the clitellum, which forms a ring around the worm. The worm wriggle out of the ring and during this process, injects eggs and sperms to 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.

Other Features

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 a function of directing the body movement with respect to light. There is no skeleton in earthworms.

Earthworms may appear as tiny creatures with long tube-like body with tapering ends, but, in fact, they are highly evolved worms, helpful to mankind in many ways.