Category: Annelid worms

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The Annelida (from Latin "annellus", meaning little ring) are a large animal phylum comprising segmented worms, most familiar as the earthworm. They are characterised by a combination of features, the most obvious of which is a long body composed of nearly identical segments containing a standard set of organs. There are more than 17,000 modern species living in marine, freshwater and moist terrestrial environments, including the ragworms and leeches. Traditional taxonomy divided the annelids into three major groups: the mainly marine polychaetes; oligochaetes, which include earthworms; and Hirudinea or leech-like species, many of which are parasitic. However, this taxonomy has been substantially revised over the last decade on the basis of molecular phylogenetic analyses. For example, the tiny archiannelids have been found to belong within the polychaetes. Enigmatic worms with their reduced body form, such as myzostomids, are still controversial in terms of phylogeny but may now be classified as annelids.

In terms of convergences, annelids are instructive in a number of instances. Most notable are luminescence, moulting and the independent evolution of both compound eyes (especially in sabellids) and camera eyes (in alciopids). Within the group, there are also some interesting examples of convergent evolution. The polychaete Hrabiella, which has unusually adopted a terrestrial lifestyle, shows many oligochaete-like features. Initially considered a convergence due to similar selection pressure, some more recent work has suggested that this genus could actually belong to a sister-group of oligochaetes. Another very remarkable convergence is the evolution in some oligochaetes of love darts, very similar to the more familiar examples in snails and slugs, which are involved in the injection of allohormones probably manipulating sperm competition. A number of annelids build calcareous tubes, of which the small spiral-like forms referred to as spirobids are well known. The spirobid morph, however, is most likely convergent, and it is far from clear that a number of fossil examples are actually the result of tube-building by annelids, let alone polychaetes.

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Flying through the air on a summer's evening or sparkling in the ocean you may see magical flashes of light that signal some of nature's most enchanting creatures, those that are bioluminescent.

Deep sea communities: cold seeps and hot vents n/a Not Available
Agriculture in marine polychaete annelids

Some polychaetes attach pieces of algae to their dwelling tube. Just for decoration? No, but for a much more substantial (and convergent) benefit...

Vibrational communication in animals

What on earth could an elephant or treehoppers have in common with a seismometer?

Compound eyes in sabellid annelids

Compound eyes have evolved convergently in the annelids, notably amongst the sabellids, where they evidently serve as an optical alarm system.

Camera eyes in alciopid annelids

There is a striking example in the group known as the alciopids, which are pelagic polychaetes. The similarity of their camera eye to the vertebrate eye has attracted considerable comment.

Agriculture: from ants to dugongs

Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae…

Transparent tissues: eyes, bodies and reflective surfaces

Read on if you want to know about the numerous animal equivalents to the invisible man...

Love darts in slugs, snails and annelid worms

The curious habit of stabbing their partners with sharp calcareous (or chitinous) darts during courtship and prior to actual copulation has understandably attracted considerable attention.

Worm-like body form

Man is but a worm, but so are many other vertebrates...

Annelids: insights into convergence

Notable instances of convergence involving annelids include luminescence, moulting and the independent evolution of both compound eyes (e.g. in sabellids) and camera eyes (in alciopids).

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Burrowing: from worms to vertebrates

Quite a few adaptations are useful for burrowing into the soil. So it is not exactly surprising that they have evolved several times...

Moulting in arthopods, annelids and other animals

Moulting has, however, evolved independently in other groups, including the annelids where some polychaetes shed their jaws.

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Camera eyes in vertebrates, cephalopods and other animals

Camera eyes are superb optical devices, so it is not surprising that they have evolved several times. But why, of all animals, in the brainless jellyfish? Or for that matter in a slow-moving snail?