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Annelids are a phylum of invertebrate worms. They are the segmented worms, with over 17, known species. Well known species are earthworms and leeches. Annelids can be found in most wet environments. Some of these species are parasitic or mutualistic.
Table of contents


  1. What is the Basic Body Plan of an Annelid??
  2. The Star-Chamber, Volume 2 An Historical Romance.
  3. Introduction?
  4. Walk by Faith.

However, biological classification of annelids can vary widely among taxonomists. Some consider there to be three classes of annelids: Polychaeta, Clitellata, and Aelosomata. The Clitellata are then further divided into three or four subclasses: Oligochaeta earthworms and freshwater worms , Hirundinea leeches , and Branchiobdella about species of small animals that are largely parasites or commensals on crayfish , and sometimes Acanthobdellida leech-like, temporary parasite, which is also placed in Hirundinea in some classifications.

Aelosomata includes small to very small annelides, with about 25 known species. This taxonomy looks like the following:. Another taxonomic scheme regards two groups of polychaetes—the Archiannelida and the Myzostomaria —as classes in their own right, and recognizes four total classes: Polychaeta, Clitellata, Myzostomida, and Archiannelida. This looks like the following:.

In some biological classifications, the Clitellata is considered a subphylum and the Oligochaeta, Hirudinea, and Branchiobdellida are treated as classes of this subphylusm. A simple classification scheme is to recognize two classes of annelids, the Polychaeta and the Clitellata:.

Breadcrumb

There have also been proposals to consider the Clitellata as part of the Polychaeta, thus making the latter term synonymous with the annelids. Annelids are triploblastic protostomes with a coelom at least historically , closed circulatory system, and true segmentation. Protosomes are animals with bilaterial symmetry where the first opening in development, the blastophore, becomes its mouth. Triploblastic means that they have three primary tissue areas formed during embryogenesis.


  • Annelids - Worms with Segments.
  • Annelida: Annelid Characteristics;
  • Introduction to the Annelida.
  • annelid - Wiktionary!

    • A coelom is a fluid-filled body cavity. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coelom is divided into a sequence of compartments by walls called septa.

    In the most 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. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals.

    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 quite variable but is usually specialized. For example, in some groups notably most earthworms it has a typhlosole internal fold of the intestine or intestine inner wall , to increase surface area, along much of its length.

    Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers. The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.

    The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment.

    Annelids: Powerful and Capable Worms | the Shape of Life | The Story of the Animal Kingdom

    Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units; in others, the number of segments is fixed in early development. Depending upon the species, annelids can reproduce both sexually and asexually. Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly.

    Background

    The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus , for example, are known to reproduce by the body breaking into such fragments. Many other taxa such as most earthworms cannot reproduce this way, though they have varying abilities to regrow amputated segments. Sexual reproduction allows a species to better adapt to its environment. See More First Known Use of annelid , in the meaning defined above History and Etymology for annelid ultimately from Latin anellus little ring — more at annulet Keep scrolling for more Learn More about annelid Share annelid Post the Definition of annelid to Facebook Share the Definition of annelid on Twitter Time Traveler for annelid.

    Accessed 8 January More from Merriam-Webster on annelid Britannica. Please tell us where you read or heard it including the quote, if possible.

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    • Boneyard Awakenings.
    • Introduction to the Annelida;
    • General features.
    • Introduction!

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      Annelids: Powerful and Capable Worms

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