Annelids

Annelids, or segmented worms, are segmented bilaterian invertebrates that are very important to a variety of marine and terrestrial environments. All annelids have a central body cavity called a coelom, bristles called setae, and segments called annulations which they are neamed after. There are three major groups of of annelids; the class Polychaeta which are almost entirely marine in nature and the subclasses of Oligochaeta which are earthworms and their relatives, and Hirudinea which are leeches. Among these groups there are approximately 17,000 described species. Many of the species of annelids reproduce sexually and are hermaphroditic, but some are able to asexually reproduce. Annelids are found across the entire planet in almost every kind of environment imaginable. Annelids are generally soft tissue organisms, but there is evidence of them in the fossil record dating back to the Ordovician period.

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Distribution and Habitat

Annelids, due to their incredible diversity, live in nearly every habitat on planet, however they have no means of protecting themselves from desiccation, therefore they prefer to live in wet environments. These environments include oceanic sea floors, freshwater systems, and damp soil.[3] The class Polychaeta are primarily benthic organisms that can live in saline, brackish, or freshwater environments. Their distribution in these areas are primarily controlled by the space available to them, the dissolved oxygen content in the water, the rate of movement of the water, and the relative salinity and temperature of the water to the Polychaeta.[4] Oligochaetes are primarily found in the soil but a few are found in aquatic environments, and leeches are almost entirely aquatic or limited to humid areas.[3]

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