Earthworm Biology

What are Earthworms?

The term Earthworm refers to a specific group of invertebrates within the taxonomic phylum Annelida. Earthworms belong to the Oligochaetes-which means 'few bristles' and are represented by 27 species of earthworm here in the UK. Worldwide there are over 3,000 species described- and advances in taxonomy using DNA is changing this, highlighting new species and new subspecies. Earthworms are, as their name suggests, terrestrial, their skin is permeable and they need a moist environment so they don't dry out. The different species of earthworms have individual requirements just as a dog has from a cat, some earthworm species live in compost, some live in permanent burrows deep deep down in the soil, others are content with the middle ground and make complex networks of tunnels as they explore the earth. There is still a lot we don't know about them, and the Earthworm Society of Britain aims to improve this!

Earthworm structure

An earthworm consists of a digestive tube housed within a thick cylindrical muscular tube that forms the body. The body is divided into segments, and furrows on the surface of the body mark the division between each segment.

The first segment encloses the mouth, and has a fleshy, muscular lobe on the top. This lobe can be pulled in to seal the mouth, or extended forward to probe the immediate surroundings. All segments, except the first, have eight retractable bristles which help the earthworm to grip surfaces as it moves.The picture below, taken from Key to the Earthworms of the UK & Ireland by Sherlock (2012), shows some of the internal features of an adult earthworm.

Earthworm anatomy illustration

Earthworm reproduction

Earthworms mating
Earthworms mating. Picture by Dr Kevin Butt, UCLAN, with thanks.

 

Earthworm egg from soil
Earthworm egg in soil.

Earthworms are hermaphrodites, meaning they have both male and female reproductive organs.

When two earthworms are ready to mate they adopt a head-to-tail position, cover themselves in a layer of mucus, and exchange sperm. The saddle produces a mucous tube which detaches and moves forward along the body, collecting on the way the earthworm’s own eggs and the sperm received from its partner.

Fertilization occurs in the mucous tube which is shed from the front end of the earthworm. This dries in the soil to become an egg capsule, from which one or more young earthworms will eventually hatch. Many species can reproduce several times a year.