Sense Organs of Earthworm

Last Updated on September 18, 2020 by Sagar Aryal

  • Earthworms have well-developed sense organs or receptor organs.
  • Simple in structures.
  • Reacts to a number of stimuli with the help of 3 types of sense organs: epidermal receptor, buccal receptor, and photoreceptor.

Sense organs of Earthworm

Figure: Sense organs of Earthworm. Image Source: Studio Biology.

Epidermal receptors

  • Distributed all over the epidermis but more abundant on lateral sides and ventral surface of the body.
  • They consist of an ovoid tall, slender receptor cell in the epidermis which causes an elevation of the cuticle.
  • These cells are separated from each other by space.
  • Each cell has a nucleus at different levels and posses internally a few basal cells.
  • The receptor cells bear a small hair-like process at their outer ends which penetrate the cuticle and project beyond it.
  • And connected nerve fibers at the inner ends.
  • They are tactile (relating to touch) in function. i.e., tangoreceptors.
  • According to some, they also respond to chemical and thermal stimuli and change in temperature.
  • Hence, earthworms are very sensitive to touch and vibrations transmitted through solid objects, though they cannot hear at all.

Buccal receptors

  • Found in the epithelium of the buccal cavity in large numbers.
  • Similar to epidermal receptors they consist of a group of tall cells which project beyond the epithelial cells except
  • They possess broader outer ends.
  • They have better sensory hair-like processes.
  • Their nuclei lie below the middle parts of the cells.
  • These receptors serve to smell i.e., olfactorecptors and taste food i.e.,
  • These cells can distinguish between the tastes of different vegetable foods but
  • Their sense of smell is poorly developed.
  • Also, respond to chemical stimuli.

Photoreceptors

  • It is a single ovoid cell in the inner parts of the epidermis.
  • Has a nucleus and cytoplasm with a network of neurofibrillae and small transparent L-shaped lens or optic cell or Phaosome
  • These cells are curved and made up of hyaline substances.
  • One or two nerve fibers enter this optic cell.
  • The lens focuses light rays from all directions on neurofibrils.
  • Neurofibrils converge to an afferent nerve fiber which leaves the cell at its base to join the central nervous system.
  • Restricted only to the dorsal surface and are more numerous on prostomium and peristomium and gradually decrease in number towards the posterior end of the body.
  • Totally absent in clitellum.
  • These receptors are sensitive to light and also called little eyes or
  • The network of photoreceptors is formed by branching and rebranching of the nerve fibers entering into it and characteristically called
  • Photoreceptors enable the worm to judge the intensity and duration of light.
  • The earthworm shows a negative response to even low light.
  • Hence, they retreat into their burrow at daytime and come out at night i.e., nocturnal in habitat.

Video Lecture: Sense Organs in Earthworm By Studio Biology.

References and Sources

  • Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  • Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
  • 7% – https://www.biologydiscussion.com/invertebrate-zoology/earthworms/pheretima-habit-and-habitat-and-external-features/29340
  • 2% – https://www.shareyouressays.com/knowledge/biology-question-bank-144-mcqs-on-animal-kingdom-answered/114611
  • 1% – https://www.answers.com/Q/Why_do_earthworms_come_out_at_night
  • 1% – https://askabiologist.asu.edu/smell-taste

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