Overview of Earthworm Anatomy
Earthworms are invertebrates meaning they do not have backbones such as reptiles, mammals, birds, fish, and amphibians. Examples of invertebrates insects, spiders, jellyfish, and millipedes.
Earthworms belong to a group of animals called Annelids. Annelids are segmented worms or worms with little rings around their bodies. The segments of the worms are well-defined - of similar size. They are separated from each other by septa (membranous partition). The exceptions are the head and tail regions which do not look alike.
An earthworm has about 100-150 segments each having muscles and bristles called setae. The setae help to control the movement of the worm through the soil. Furthermore, bristles hold firmly into the ground a section of the worm as the other part of the body protrudes forward.
The digestive system of earthworm consists of pharynx, the esophagus, the crop, the intestine, and the gizzard.
When an earthworm eats food such as leaves through its mouth, it is grabbed by the pharynx. The pharynx pulls the food back into the mouth where it is mixed with saliva for easier swallowing. The food moves to the esophagus which contains calciferous glands. The glands release calcium carbonate whose task is to get rid of excess calcium in the earthworm's body.
The food then moves to the crop where it is stored. The crop acts as a storage compartment for food. From the crop, food moves into the gizzard.
In the gizzard, stones that had been swallowed are used by the strong muscles of the gizzard to grind the food. The food is moved into the intestine where it comes into contact with digestive juices that act as an aid in the digestion process by breaking the food into finer particles.
The wall of the intestine contains blood vessels. The digested food is absorbed in the blood and transported to the rest of the body.
The circulatory system of an earthworm consists of three main blood vessels: the aortic arch, dorsal and ventral. The aortic arch acts like a human heart. The aortic arch pumps blood into the dorsal and ventral blood vessels. The dorsal blood vessels transport blood to the front of the earthworm's body while the ventral blood vessels transport blood to the back of the body.
Earthworms breathe through the skin since they don't have lungs. This is only possible through diffusion. For diffusion to occur, the earthworm's skin needs to be moist. This is the reason earthworms appear on the surface of the soil during the night when it cooler – there is low evaporation. Normally, the epidermis (skin) of earthworm releases mucous to keep the skin moist.
An earthworm does not have ears, nose, eyes or hands to help sense the environment around it. It uses its skin as a sensory receptor. Some species of earthworms use prostomium which is a tongue-like lobe found above the mouth of the earthworm. Prostomium acts the same function as the skin the earthworm - to sense the environment.