Earthworms are tubular animals with specific bodily organs taking care of different life functions. Within their tubular bodies are a digestive system, a circulatory system complete with multiple heart-like organs, a simple nervous system and a reproductive system. The body of earthworms is segmented with the anterior segments being more specialized than the posterior segments.
- Digestive System – While most people cannot tell the difference between the anterior and posterior ends of an earthworm, the anterior end indeed has a mouth, and near the anterior end is an anus. Some earthworms feed on plant matter, but other species of earthworms are geophagous, which means they eat dirt, mud or clay, and nutrients are extracted from it by the digestive system. The stomach of earthworms is called the crop. Food is stored in the crop until it travels through the gizzard where it is churned to expand its surface area, making the nutrients more available for absorption. Finally, the food moves through the intestine where the nutrients are absorbed before passing out through the anus.
- Circulatory System – The circulatory system of earthworms is composed of two primary blood vessels that extend through the entire length of the body. Blood moves through the ventral blood vessel to the posterior of the body and through the dorsal vessel to the anterior of the body. The dorsal blood vessel contracts to push the blood back and then forward to the ventral vessel where a series of aortic arches act as hearts to move blood forward to the anterior end and through the capillaries.
- Nervous System – The earthworm’s nervous system is controlled by its cerebral ganglion, which functions as a simple brain. The ventral nerve cord is attached to the ganglion in the anterior segments, and it runs through the length of the body. In each segment of the body, segmental ganglion attach to the ventral nerve, forming a complete nervous system. The segmental ganglions’ primary responsibility is movement, but these nerves are also sensitive to touch and temperature.
- Reproductive System – Earthworms are hermaphroditic, which means they have both male and female reproductive organs. However, earthworms reproduce sexually. During copulation, a pair of earthworms exchanges sperm. The sperm is stored in sacs called spermathecae, until it can be transferred to an egg. In some species, this is done externally. The earthworm secretes a cocoon in which it deposits its eggs and its mate’s stored sperm. The baby earthworms emerge from the cocoon nearly fully developed but very small. They grow to full-size over the course of approximately one year.