Kinds of receptors found in the sense organ of leaches are given below:
1. Nerve endings
2. Annular receptors
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3. Segmental receptors
4. Eyes.
1. Free nerve endings:
These are found all over the body between epidermal cells. They act like chemoreceptors by detecting chemical changes in the surrounding water.
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2. Annular receptors:
In each annulus of the body 36 annular receptors are found, which consist of many flattened overlapping cells that receive their nerve supply from the lateral branches. They act like tangoreceptors or tactile organs.
3. Segmental receptors:
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They are found as 4 pairs dorsally, 3 pairs ventrally in the first annulus of each segment. Each receptor consists of two types of cells
(i) tactile cells or tangoreceptors and
(ii) Light sensitive cells or photo receptors.
Thus they function both as tango receptor and photoreceptor.
4. Eyes:
These are 5 pairs; appear as a semicircle of black spots. One pair of eyes in the first annulus of each of the first five segments.
The wall of cup-shaped eyes is formed of black pigmented tissue enclosing a large number of clear, retractile photoreceptor cells arranged in longitudinal rows.
Each cell contains nucleus and cytoplasm that is surrounded by a crescentic hyaline lens. Outer free surface is covered by transparent epidermal cells and cuticle forming a sort of cornea. Each eye is supplied by optic nerve.
Eyes are differently directed. Each eye can receive light only from one direction. It is not known whether eyes are able to form any image or not.
Probably the only enable the leach to differentiate light from the darkness and to locate the direction of source of light.