Sense Organs of Earthworm

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.

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

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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.

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.

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.

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