Deseribe the structure of Compound eye of Palacmon and point
out the mechanism of image formation.
Ans. Eye: Each movable and stalked eye is compound in nature, í.e
made up of several simple visual clements. Each component is called an
ommatidium or ocellus.
a) Structure of an Ommatidium:
Each ommatidium is divisible into two part- outer dioptrical region fors
focussing and inner retinal part for receiving light stimuli. These two parts in
each ommatidium contain following parts from outer to inner sides
(i) Cornea :It is the outermost transparent cuticular layer. The cornea of
all the ommatidia gives the outermost part of the eye a graph paper like
appcarance and each square is called a facet.
ii) Corncagen Cells:Immediately beneath the cornea a pair of corneagen
cells is present which are responsible for the replacement of cornea.
iii) Crystalline Cone: This is an elongated transparent body, placed
beneath the corneagen cells and works as lens.
(iv) Cone cells or Vitrellae These cells are four in number and they
encircle the cone or lens to provide nourishment.
(v) Rhabdome Elongated transvesely striated body which is situatcd
immediately beneath the cone cells.
(vi) Retinular cells : These are elongated sickle - shaped cells. Seven
such cells secrete the rhabdome and encircle it to provide its nutrition.
(vii) Pigmcat sheath :Two separate sheaths containing chromatophores
are responsible for scparating one ommatidium from the other. The group of
pigment shcath which is present around cone and cone cells is called iris sheath
While the other group around rhabdome and retinular cells is called retinal sheath. Pigment sheaths are able to contract and relax, which depends upon
the intensity of light.
Fig. A. Compound eye of Palaemon (surface view)
B. Compound eye of Palaemon (Longitudinal) section, diagrammatic.
C. Histological structure of four ommatidia (longitudinal section
diagrammating).
(b) Mechanism of Image Formation:
The ommatidia may work singly
or collectively. During bright light both the pigment sheaths extend and
completely separate the ommatidia, which results into the formation of a large
number of images. This type of vision is called opposition image or mosaic
vision. When light is dim, pigment sheath contracts and all the ommatidia
work together to form a single but blurred image. This kind of vision is known
as superposition image. The prawn can move its eye considerably and has
nearly 360° vision
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