Annelid

The annelids, collectively called                      Annelids
Annelida, are a large phylum of animals,      Scientific classification
comprising the segmented worms, with
about 15000 modern species including the  Kingdom:  Animalia
well-known earthworms and leeches. They   Phylum:   Annelida
are found in most wet environments,
including many terrestrial, freshwater,         Classes and subclasses
and especially marine species, as well as Class Polychaeta - paraphyletic?
some which are parasitic or mutualistic.  Class Clitellata
They range in length from under a         ΚΚΚ Oligochaeta (e.g. earthworms)
millimetre to over 3 metres.              ΚΚΚ Acanthobdellida
                                          ΚΚΚ Branchiobdellida
Annelids are triploblastic protostomes.   ΚΚΚ Hirudinea (leeches)
The body cavity is a coelom, which is     Class Myzostomida
divided into a sequence of compartments   Class Archiannelida - polyphyletic
by walls called septae. In the most       Class Echiura
general forms each compartment
corresponds to a single segment of the body, which also includes a portion
of the nervous and (closed) circulatory systems, allowing it to function
relatively independently. Each segment is marked externally by one or more
rings, called annuli. In most forms they also carry a varying number of
bristles, called setae, and among the polychaetes a pair of appendages,
called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which
carries the mouth, and posterior to them lies the pygidium, where the anus
is located. The digestive tract is usually specialized. Different species of
annelids have a wide variety of diets, including active and passive hunters,
scavengers, filter feeders, direct deposit feeders which simply ingest the
sediments, and blood-suckers.

Most polychaete worms have separate males and females. The earliest larval
stage, which is lost in some groups, is a ciliated trocophore, similar to
those found in other phyla. The animal then begins to develop its segments,
one after another, until it reaches its adult size. The oligochaetes and
leeches tend to be hermaphroditic and lack free-living larvae of this sort.
Many annelids have some regenerative abilities, often to the point where
each half of an adult divided cross-wise will survive.

The annelid fossil record is sparse, but a few definite forms are known as
early as the Cambrian, and there are some signs they were around in the
later Precambrian. A few small groups have been treated as separate phyla:
the Pogonophora and Vestimentifera, now included in the family Siboglinidae,
and the Echiura.

The arthropods and their kin have long been the closest relatives of the
annelids, on account of their common segmented structure, but a number of
differences between the two groups suggest this may be convergent evolution.
The other major phylum which is of definite relation to the annelids are the
molluscs, which share with them the presence of trocophore larvae. These
groups are united as the Trochozoa, and when the arthropods are included,
they and the annelids are treated in a subgroup called the Articulata.

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