Earthworms need moisture to survive and consequently commonly live underground in damp soil. They are cylindrical in shape, and their body structure is surprisingly simple with a muscular outside body that lines their digestive tract and circulatory system.
This circulatory system is very simple, with only two blood vessels (the dorsal and the ventral) which run between the anterior and posterior of the creature, with blood being pumped by aoertic arches in the case of the ventral vessel, or moved back to the anterior by the dorsal vessel contracting.
However, although simple, earthworms do display distinct segments, which are more specialised towards the anterior (head). Consequently, segments situated further back in the body can be regenerated in many cases, but it is dependant on the type of species and the actual extent of damage.
Earthworms are also hermaphrodites, holding both male and female sexual organs, however they commonly mate and then store the other individual’s sperm for reproduction.