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Segmented worms (Annelida) are a group of invertebrates that includes about 12,000 species of earthworms, ragworms, and leeches. Segmented worms live in marine habitats such as the intertidal zone and near hydrothermal vents. Segmented worms also inhabit freshwater aquatic habitats as well as moist terrestrial habitats such as forest floors.
Anatomy of Segmented Worms
Segmented worms are bilaterally symmetrical. Their body consists of a head region, a tail region, and a middle region of numerous repeated segments. Each segment is separate from the others by a structure called septa. Each segment contains a complete set of organs. Each segment also has a pair of hooks and bristles and in marine species a pair of parapodia (appendages used for movement). The mouth is located on the first segment at the head-end of the animal and the gut runs through all segments to the end where an anus is located in the tail segment. In many species, blood circulates within blood vessels. Their body is filled with fluid that gives the animal shape through hydrostatic pressure. Most segmented worms burrow in terrestrial soils or sediments at the bottom of freshwater or marine waters.
The body cavity of a segmented worm is filled with fluid inside which the gut runs the length of the animal from head to tail. The outer layer of the body consists of two layers of muscle, one layer that has fibers that run longitudinally, a second layer that has muscle fibers that run in a circular pattern.
Segmented worms move by coordinating their muscles along the length of their body. The two layers of muscles (longitudinal and circular) can be contracted such that parts of the body can be alternately long and thin or short and thick. This enables the segmented worm to pass a wave of movement along its body that enables it to, for example, move through loose earth (in the case of the earthworm). They can make their head region thin so that it can be used to penetrate through new soil and build subterranean burrows and paths.
Many species of segmented worms reproduce asexually but some species reproduce sexually. Most species produce larvae that develop into small adult organisms.
Most segmented worms feed on decaying plant materials. An exception to this are the leeches, a group of segmented worms, are freshwater parasitic worms. Leeches have two suckers, one at the head end of the body, the other at the tail end of the body. They attach to their host to feed on blood. They produce an anticoagulant enzyme known as hirudin to prevent blood from clotting while they feed. Many leeches also ingest small invertebrate prey whole.
The beard worms (Pogonophora) and the spoon worms (Echiura) are considered to be close relatives of the annelids though their representation in the fossil record is rare. The segmented worms along with the beard worms and spoon worms belong to the Trochozoa.
Segmented worms are classified within the following taxonomic hierarchy:
Animals > Invertebrates > Segmented Worms
Segmented worms are divided into the following taxonomic groups:
- Polychaetes - The polychaetes include about 12,000 species that are characterized by having multiple hairs on each segment. They have nuchal organs on their neck that function as chemosensory organs. Most polychaetes are marine animals although some species live in terrestrial or freshwater habitats.
- Clitellates - The clitellates include about 10,000 species that have no nuchal organs or parapodia. They are noted for their clitellum, a thick pink section of their body that produces a cocoon to store and feed fertilized eggs until they hatch. The Clitellates are further divided into the oligochaetes (which include earthworms) and the Hirudinea (the leeches).