Annelida: worms and leeches

Characteristics
Annelids can be immediately distinguished from most other invertebrates by their external body structure. Annelids are divided into 3 groups, 2 of which are terrestrial and include earthworms (class Oligochaeta) and leeches (class Hirudinidea). The third group, Polychaete worms occur in marine environments. Annelids can be distinguished from most other invertebrates by the following characteristics:

• Long cylindrical shaped bodies made up of many similar segments

• No appendages

• No antennae

• No obvious head end

The Oligochaetes are a group that consists of terrestrial earthworms and species that live in freshwater environments. Earthworms range in size from a few centimetres up to 2-3 metres in length such as the Australian native giant Gippsland earthworm. Native earthworm species are often eradicated from natural areas as people clear native vegetation and introduced species become more dominant in these disturbed habitats. Introduced earthworms are most common in disturbed environments such as suburban gardens and farmland paddocks.

Life Cycle
Earthworms are hermaphrodites, which means that each animal has both male and female reproductive organs in the same body. Earthworms lay their eggs in a cocoon that is then deposited in the soil. On hatching the young worms resemble small adults and will grow continually until they reach maturity.

Feeding
Most earthworms feed on a wide variety of organic matter, primarily detritus and algae. Earthworms are considered to be beneficial animals because as they move through the ground feeding, they ingest huge quantities of soil accelerating the nutrient cycle and increasing its productivity. As earthworms feed they pass out what they do not need in the form of castings. These castings are high in nutrients and are in turn food for other animals. Microorganisms also feed on the castings, breaking them down and releasing nutrients that can be used by plants. The burrowing habits of earthworms are also beneficial, as they aerate the soil and allow rainwater to penetrate deeper into the ground.

Habitat
Earthworms are found in many terrestrial environments ranging from semiarid habitats to moist, well-vegetated areas. Earthworms are more common in wetter areas and can be found in soil, leaf litter and under rocks and logs. Many are often found on the surface after heavy rain, moving out of the soil as it becomes waterlogged.