Summary
Segmented worms like earthworms and leeches with a closed circulatory system.
In depth
Phylum Annelida. Metameric segmentation allows specialised segments. Setae aid movement; hydrostatic skeleton and closed circulation with hearts (aortic arches). Earthworms aerate soil.
Segmentation allows efficient movement. Polychaetes include marine worms; oligochaetes include earthworms. Closed circulation and nephridia excrete waste.
Key idea: metamerism is repeated body segments.
Segmented worms include earthworms, leeches and polychaetes (mostly marine). Segmentation (metamerism) allows repetition and specialisation of body parts and better movement; each segment has its own coelom compartment, nerve ganglia and excretory organs (nephridia). Earthworms are hermaphrodites with a clitellum, and they swallow soil, aerating and enriching it; Darwin studied their effect, estimating they bring tonnes of soil to the surface per hectare each year.
Leeches secrete hirudin, an anticoagulant used in medicine after microsurgery. Polychaetes include fanworms and giant tube worms near vents (Riftia) that depend on chemosynthetic bacteria. Annelids have a closed circulation with blood containing haemoglobin.
Frequently asked questions
What is Annelids?
Segmented worms like earthworms and leeches with a closed circulatory system.
Which area of biology covers Annelids, and how advanced is it?
Annelids is one of the 53 topics in Zoology & Animal Diversity. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Flatworms, Arthropods, Body Cavities.
Why does Annelids matter?
Earthworm activity improves soil structure and fertility.
Can you give an example of Annelids?
Leeches secrete hirudin, an anticoagulant used medically.
What should I learn before Annelids?
Annelids is a Beginner topic, so no earlier topic is required. Other Beginner topics in Zoology & Animal Diversity that pair well with it include Animal Kingdom, Vertebrates and Invertebrates, Sponges.
What topics are related to Annelids?
The closest topics to Annelids are Flatworms, Arthropods, Body Cavities, Animal Excretion. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Annelids?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Annelids. To practise the whole of Zoology & Animal Diversity, use the Zoology & Animal Diversity quiz.
How is Annelids different from Flatworms?
Annelids: Segmented worms like earthworms and leeches with a closed circulatory system. Flatworms: Soft-bodied, bilaterally symmetric worms with no body cavity, including tapeworms and flukes. Read both side by side and note what each one explains.
How should I study Annelids step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Flatworms and explain the difference aloud. Finally, take the Zoology & Animal Diversity quiz and come back to this page for any question you miss.