The following points highlight the four main types of sense organs in Palaemon Malcolmsonii. The sense organs are: 1. Compound Eyes 2. Statocysts 3. Tactile Organs 4. Olfactory Setae.
Sense Organ: Type # 1. Compound Eyes:
In Palaemon Malcolmsonii, there is a pair of black and hemispherical compound eyes. Each eye is borne on a short two-jointed movable stalk lying in an orbital notch.
Each eye is a composite structure made of a large number of structural and functional visual units called ommatidia or ocelli lying radially. This type of eyes, made up of hundreds or thousands of ommatidia, are termed as the compound eyes and are found in majority of arthropods.
The eye is covered with a transparent chitinous covering of cuticle forming a cornea. The cornea is divided into a large number of square facets placed in juxtaposition like squares of a graph paper. Each facet corresponds to a single ommatidium and below each facet lies one ommatidium inside the eye. All ommatidia are simple and are arranged radially lying side by side and separated by dark pigment cells.
Structure of ommatidium:
Each ommatidium is composed of a number of cells arranged end to end along a central axis.
However, it comprises the following structures:
The outermost layer of an eye is the transparent cuticle forming cornea which is divided into a large number of square-like facets. These facets are thickened in the centre to give them the appearance of a biconvex lens. Thus, each corneal facet behaves like a lens and sheds off at the time of moulting, again secreted by the underlying cells.
(ii) Corneagen cells:
Each corneal facet is followed by a group of two cells; these cells are modified epidermal cells called corneagen cells. Their function is to secrete cornea when it is moulted off.
(iii) Cone cells or vitrellae:
These are a group of four much elongated cells situated beneath the corneagen cells. These cells secrete and enclose a transparent and refractile crystalline cone which works like a second lens. The inner end of cone cells is long and tapering. The cornea, corneagen cells and cone cells together constitute the dioptrical region, whose function is to focus the light rays on the inner sensitive region.
(iv) Retinal cells:
The cone cells are followed by a group of seven elongated cells forming the proximal part of the axis of an ommatidium. These cells are elongated and provided with distally placed dilations having nuclei.
(v) Rhabdome or optic rod:
It is an elongated, spindle-shaped and transversely striated body which is secreted and fully enclosed by the retinal cells.
(vi) Basement membrane:
It is the innermost layer of a thin fenestrated membrane that marks the internal boundary of the ommatidia in a compound eye. The ommatidia are innervated by optic nerve fibres, coming from optic ganglia, through the fenestrae in the basement membrane. The retinal cells and rhabdome up to the basement membrane constitute the receptor region; its function is to receive the light rays focused by the dioptrical region.
Each ommatidium is cut off from its neighbouring ommatidia by a sheath of dark pigment formed by the surrounding amoeboid chromatophores which are arranged in two groups. The proximal group surrounding the rhabdome forms the retinal pigment and the distal group surrounding the crystalline cone forms the iris pigment. The amoeboid pigment cells take up different positions according to the changes in the intensity of light.
Working of the compound eye:
The working of the compound eye is very complex. In the formation of an image, several adjacent ommatidia take part and light enters through them.
Each ommatidium is capable of producing a separate image of a small part of the object seen. Therefore, the whole image formed in a compound eye is actually made of several small pieces contributed by the several adjacent ommatidia. On this account the vision effected through a compound eye is called mosaic vision.
In diurnal crustaceans the compound eyes are adapted for bright light and it produces an apposition or mosaic image. But in nocturnal forms, like Palaemon, it is adapted for seeing in weak light and superpostion image is formed.
Formation of apposition or mosaic image:
In the bright light, during the day time, the pigment cells spread in such a way that they completely separate optically one ommatidium from the adjacent ommatidia.
In this condition, rays of light, which strike the cornea obliquely, are absorbed by the pigment cells, therefore, they cannot produce a visual effect. Only those rays of light, which pass directly through the centre of the cornea, can travel through the ommatidium and reach the rhabdome to from an image of a part of an object.
These small parts, placed together like the parts in a mosaic, form the image of the entire object. This is known as a mosaic vision in which the rays are received simultaneously by distinctly separate visual elements, i.e., ommatidiaand the image is made up of several components placed in juxtaposition. Such an image is called an apposition image.
The sharpness of the image depends upon the number of ommatidia involved and the degree of their isolation from one another ; the larger number of ommatidia and more complete their isolation from one another, the sharper the image.
However, an eye adapted for this type of image formation functions best at short distances only; it is, therefore, most of the arthropods are short-sighted. Such arthropods are usually night blind, e.g., butterflies.
Formation of superposition image:
In the dim light, the pigment cells migrate towards the distal and basal parts of the ommatidia and the neighbouring ommatidia work in unison. In this condition, even the oblique rays of the light are capable of forming a point of image, after passing through a number of ommatidia.
As a result, an overlapping of the adjacent points of image takes place and, thus, a continuous image is formed. Such an image is called superposition image.
In this case, the vision is not distinct but the animal is able to have some sort of idea of its surrounding objects, specially of their movements.
In some insects, like fire-flies and some moths, the eyes are permanently set in the way that they are adapted for vision in the dim light, i.e., at night but they are day-blind, e.g., moths and fireflies. It is probable that the Palaemon Malcolmsonii like most of the arthropods can adjust its eyes so as to form both the types of images according to the intensity of light available.
Sense Organ: Type # 2. Statocyst:
The statocysts are a pair of small white, cuticular, hollow, sub-spherical structures. Each statocyst lies inside the precoxa of each antennule, attached to its dorsal wall. The base of each statocyst is slightly depressed and the neck opens to the outside by a minute aperture situated on the concave roof of the precoxa of antennule.
The opening is covered by a small fold of integumefit. The statocyst is innervated by a small statocystic branch of the antennulary nerve. On cutting a section of the statocyst, its cavity is found full of minute sand-particles. On removing the sand-particles, it is found that there is an oval ring of elongated delicate receptor setae, which are attached to the inner wall of the statocyst in an oval outline.
Each receptor seta consists of a swollen base and a filamentous shaft which is sharply bent about the middle of its length. The base of seta is attached to the wall of the sac by a very thin arthrodial membrane and the shaft bears fine bristles beyond the bend. Each receptor seta is innervated at its base by a fine branch of statocystic nerve.
The statocysts perceive the direction of the force of gravity and function as the organs of orientation and equilibrium. The sand particles function as statoliths. With the change of the position of prawn in water, the sand particles press on the receptor setae and stimulate them.
The stimulated setae convey the information to the brain through the nerves and, thus, animal becomes aware of its position and maintains it. The function of the statocysts is to maintain the balance and position. In moulting of prawn, the lining of the statocyst and its statoliths are cast off and then renewed.
Sense Organ: Type # 3. Tactile Setae:
Tactile setae are found on the feelers of antennae and are abundant on the borders of the flattened portions of the appendages like the rami of the pleopods. Each tactile seta is a hollow cuticular outgrowth containing a slender prolongation of the ectoderm and the muscle fibres and also supplied with a nerve fibre and consists of two segments.
The proximal segment or shaft is slightly swollen at its base and is attached to the integument by a flexible membrane. The distal segment gradually tapers at its free end and is provided with a pair of linear rows of small barbs. Tactile setae are sensory to water current and to the substratum. They are stimulated only on being moved and not on being touched.
Sense Organ: Type # 4. Olfactory Setae:
Olfactory setae are found in a longitudinal groove on the small middle feeler which lies between the two feelers of each antennule.
Each seta consists of a proximal shaft which is attached to the integument by a flexible membrane, and a distal segment or blade which is bluntly rounded at its free end and covered with a thin membrane. Each seta is innervated with a single nerve-fibre from the olfactory branch of the antennulary nerve. The function of these setae is olfactory.