Class-14th | Nereis and Leech

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In this lesson we discuss Annelida - All about Nereis & Leech.


    ■ It is sexually mature form of Nereis in which transformation in anatomy and behaviour of the animal takes place during breeding season.
    ■ The anterior one-third region of body is the asexual region or atoke while the sexually mature posterior region is called epitoke and the worm with these two regions is known as heteronereis and the phenomenon of transformation of Nereis into sexual form is known as epitoky.
    ■ Heteronereis, instead of creeping about on sea bottom or living in burrows, swims actively in surface waters.
    ● Body is divisible into an anterior asexual atoke and a posterior sexual epitoke.
    ● Parapodia of posterior sexual region become larger and develop flattened leaf-like outgrowths for better respiration.
    ● Their setae are replaced by oar-shaped setae arranged in a fan-like manner for swimming. Dorsal cirri become sensitive.
    ● The eyes become greatly enlarged and more sensitive.
    ● Peristomial cirri become longer. Intestine becomes atrophied and functionless. Pygidium develops specialised sensory papillae.
    ■ Sexually mature individuals or Heteronereis swarm to the surface of seawater in the night in order to shed sperms or ova. Both males and females swim rapidly in a circle, females releasing a substance called fertilium which attracts the males and stimulates shedding of sperms, which in turn excites the females to shed the eggs. Fertilization is external in sea water.


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