Types Of organisms
The phylum annelida is an incredibly large phylum, including thousands of species. These species are divided into three classes.
Oligochaeta is made up of many various types of worms including the common earthworm Lumbricus terrestris), to the freshwater-dwelling worms such as iceworms.
Hirudineans, more commonly known as leeches, make up the second class of annelids. They are generally freshwater-dwelling creatures, but some are known to live in terrestrial and marine environments. They are also known to suck blood.
Marine worms(polychaetes) are a separate class of annelids found in marine environments such as the ocean. They are often brilliantly colored, and can either swim, crawl, or burrow in the ocean floor.
PhyLum Annelida
The Wonderful World of Segmented Worms
Fun Facts
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There are over 12,000 known species of annelids living on earth currently
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All annelids, except for leeches have small hairlike structures called setae
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Most annelids breathe directly through their body walls
Characteristics
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Bilateral symmetry
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Have bodies divided into cylindrical segments called metameres. Their body consists of a head region, a trunk, and an unsegmented portion called a clitellum
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Closed circulatory system with developed blood vessels, blood pumped by a contractive vessel above the gut.
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True tissues and organs with a primitive brain and gametes
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Body wall maintains structure of the worm, and is covered with muscles and small, hairlike structures for locomotion
Reproduction
Annelids can reproduce both sexually and asexuallyby budding or fragmentation. Among those who reproduce sexually, hermaphrodites are common, but most species have separate sexes, and reproduce by exchanging gametes, and then laying eggs.
Thermoregulation
Annelids do not actually have any tools to regulate their temperature, and must tolerate the temperature of the environment.
Respiration
Some aquatic annelids have thin gills that allow for the exchange of oxygen from the blood to the environment. However, most annelids don't have organs for breathing and just do so directly through the body wall.
Digestion/Nutrients
While leeches have a proboscis to swallow the blood of their host, most worms have a relatively simple digestive tract including a mouth, and unsegmented stomach, intestines, and then an anal orifice. Most of them filter out nutrients from water or soil they ingest.
Evolutionary History
Due to the lack of a true skeleton, as well as the lack of hard tissues, the fossil record is relatively sparse. The general consensus is that annelids most likely emerged in the precambrian era, but the fossils only date back to the cambrian era, approximately 540 million years ago. Two groups had evolved at that point, and it is debated how they are related exactly. However, DNA evidence has been helping to uncover this mystery.
Human Impact
Though they are pretty durable creatures, current farming practices, such as plowing and the use of pesticides are hurting the population of worms and annelids. In addition, fisheries and water pollution is hurting the marine environment, in which the marine worms live.