Sense Organs of Earthworm

In this sense organs of earthworm post we have briefly explained about 3 types of sensory organs: earthworm sensory organs epidermal receptor, buccal receptor, and photoreceptor.

Earthworm sensory organs are well developed. The structure of these earthworm sensory organs organs is basic, consisting of a single cell or a group of specialised ectodermal cells. Reacts to a number of stimuli with the help of 3 types of earthworm sensory organs: epidermal receptor, buccal receptor, and photoreceptor.

Earthworm Sensory Organs

Sense Organs of Earthworm

Earthworm Sensory Organs

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 possesses 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.

Earthworm sensory organs: Epidermal receptors

Buccal receptors

These receptors are restricted only to the epithelium of the buccal chamber. They are similar to the epidermal receptors except that they have broader outer ends. Also the earthworm sensory organs hairs are more deeply situated and better developed. These receptor cells have deeply seated nucleus. These receptors are gustatory and olfactory in function. They also respond to chemical stimuli.

Earthworm sensory organs: Buccal receptors

Photoreceptors

These are photo sensitive or light sensitive receptors which are restricted only to the dorsal surface of the body. They are more numerous on prostomium and peristomium but their number gradually decreases as we move towards the posterior end of the body. They are totally absent in the clitellar region.

Earthworm sensory organs: Photoreceptors

Each photoreceptor consists of a single ovoid cell with a nucleus and clear cytoplasm containing a network of neurofibrils and a small transparent L-shaped lens or phaosome, made up of a hyaline substance. 

The lens focuses the light from all directions on neurofibrils. All the neurofibrils converge to an afferent nerve fibre which leaves the cell at its base to join the central nervous system. Photoreceptors enable worms to judge the intensity and duration of light.

Further Readings

Reference

  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.