An earthworm is an invertebrate animal. An invertebrate animal is an animal without a backbone. An earthworm feeds on dead and live organic matter and is usually found in soil. It is tube shaped with neither legs nor hands and interestingly, its digestive system runs from its mouth to the anus, which is basically the length of its body. It uses its skin for respiration. There are two ways to look at the anatomy of an earthworm; external anatomy or internal anatomy.
The external anatomy of the worm includes the features of the worm which you can see with your naked eye or by the use of lens before the worm is dissected or cut open. In short, these are the features on the outside of the worm. These features are: segment, mouth, prostomium, anterior end, peristomium, clitellum, satae, sperm grooves, tubercula pubertatis, posterior, dorsal, ventral, periproct, epidermis and sperm duct opening. They have different body colours but are mostly dark red, muddy green and red-violet. The symmetry of an earthworm is bilateral. That is to say that if it was cut down on the centreline, the right side of its body would be identical to the left side.
The best way to learn about the internal anatomy of this worm would be probably dissecting it. Another way is by taking a look of diagrams which are available from various sources such as the internet, books or journals. Nonetheless, the internal organs of this interesting worm include mouth, brain, intestine, anus, gizzard, pharynx (throat), crop (stomach), esophagus, pharyngeal muscles, aortic arches, dorsal blood vessel, ganglia, and nephridia. To see these organs in a clearly labelled diagram, go to this University of Pennsylvania website.
The earthworm has some distinct systems; the digestive system, reproductive system, respiratory system and circulatory system. The digestive system is made up of the pharynx, esophagus, gizzard, intestine and crop. This system, which can also be referred to as the gut, runs from its mouth all the way to its anus as a straight tube. It is a very simple system; food goes in one end of the worm and comes out the other. This worm eats dirt, then digests plant and animal matter in the dirt and excretes the rest.
The reproductive system of an earthworm is very fascinating. Each worm has both the female and male sex organs and therefore, can produce both the sperm and eggs. That is to say, they are hermaphrodites. However, they mostly need a partner that they mate with in order to reproduce. The reproductive organs are the clitellum, ovaries and spermatheca. The respiratory system of the earthworm is also simple. Since it does not have any lungs, it breathes through its skin by means of diffusion. The reason why they are more often than not found in moist or damp soil is because they need to keep their skin moist. Otherwise, they would not be able to breathe. The circulatory system is a closed system. It circulates blood through the vessels exclusively. These vessels are aortic arches, dorsal blood vessels, and ventral blood vessels. The aortic acts the human heart while the dorsal vessels carries blood to the front of the worm and the ventral vessel carries blood to the back of the worm’s body.