Annelida
Annelida, or better known as Ring Worms, can be found in most aquatic environments as well as most terrestrial ones as well. Consisting of about 17,000 species, they aren't the largest, nor are they the smallest phylum in record. They use a method of locomotion called Peristalsis, which is the rapid contraction and release of muscles to propel them forward. Their body is made up of many segments, each containing identical organs, which is why it will survive when cut, even into multiple segments.
A proboscis worm is actually a predator, where it preys on other fish to eat. It can grow up to 2 meters in length and approximately 2cm in diameter; however, despite it's large size, it's nervous system is just like every other worm. It has a central nervous cord that runs from the brain down the entire length of the body, where it communicates important information from sensory organs and other intel, such as locomotion. It also has multiple cerebral organs that connect to the central nerve.
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Earthworms are also a kind of Annelid, and also the most common. It's the basis for all worm classification, and also has many kinds of names. Just like all it's other classifications, it shares the same kind of nervous system as the marine worm. It contains many brains running along it's segments connected through one main nerve cord, all of which transmits sensory data. It's nerve cord is also responsible for controlling reproduction organs, where it makes cocoons for mates.
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