Ommatidium & Statocyst in Arthropods (Prawn)

• Compound eye :-
Each compound eye in arthropods is a composite structure, made up of a large no. of structural & functional units called ommatidia lying radially. A single visual unit is called ommatidium.


• Structure of an ommatidium:- (Prawn)
Each ommatidium is composed of a no. of cells arranged end to end along a central axis as follows-
1. Cornea:-  Cornea is the outermost, transparent, cuticular layer of the compound eye. It is divided into a large no. of facets.  One ommatidium lies below one facet. Each corneal facet thickens in the center to form a biconvex lens.


2. Corneagen cells:- Immediately beneath the corneal facet, a pair of corneagen cells is present.
 They are responsible for the replacement of the facet.


3. Crystalline cone:-  It is a well developed, triangular, transparent body, surrounded by the cone cells.
It works like a second lens.


4. Cone cells:- Beneath the corneagen cells lie 4 elongated cone cells or vitrellae.
It constitute or nourish the crystalline cone.


5. Rhabdome:- It is an elongated, spindle shaped, transversely striated body.


6. Retinal cells:- These are a group of 7 elongated cells. The distal part of these cells are dilated containing nuclei.
Retinal cells surround, secrete & nourish the rhabdome.


7. Basement membrane:- Inner ends of retinal cells rest upon a basement membrane beyond which they are connected with sensory nerve fibres.


8. Pigment sheath:- Each ommatidium is separated from the neighboring ommatidia by a sheath of dark pigment. It is formed by surrounding amoeboid chromatophores which are arranged in 2 groups.
               The proximal group surrounding the rhabdome forms retinal pigment & the distal group surrounding the crystal cone forms the iris pigment.


• Statocysts:- (Prawn)
The statocysts are a pair of small, white, bead like, cuticular, hollow, spherical sacs.


• Position:-
A statocyst lies inside the basal segment or precoxa of each antennule, attached to its dorsal wall. It opens dorsally on the concave surface of the precoxa through a minute statocystic aperture.
A small statocystic branch of antennular nerve supplies the statocyst.


•  Structure of a statocyst:-
On cutting a section of the statocyst, its cavity is found full of minute sand particles.
 On removing them, it is found that there is an oval ring of elongated, delicate, receptor setae attached to the inner wall.
Each receptor setae consists of a swollen base & a filamentous shaft which is sharply bent about the middle of its length. The shaft bears fine bristles beyond the bend.
Each receptor seta is innervated at its base by a fine branch of statocystic nerve.


• Function:-
Statocyst perceives the direction of force of gravity & functions as the organ of  orientation & equilibrium.
The sand particles  serve as statoliths. Any change in the position of the swimming prawn causes a corresponding displacement of sand particles. This displacement press against some of the sensory setae & stimulate them.
Stimulated setae convey the information to brain through nerves, so that the animal corrects its position.