The pigment allows light to reach the receptors only from above and slightly to the front, but image formation does not occur. Bowl-shaped, black pigmented ocelli containing clusters of photosensitive cells are present in planaria (Fig. Isolated light sensitive cells are scattered over the body surface in earthworm. Clusters of simple eyes or ocelli in the jelly fishes help them to orient to light.
#Zoo s in organ skin
In the metazoan, portions of the skin are specialised to form eyes. 14.5) is present near the base of the flagellum. In Euglena, the photoreceptor, stigma, simply a mass of granules (Fig. Even the protozoon, in which such an organelle is lacking, respond to light and move away from bright light. Photoreceptors in different degrees of perfection are present in the majority of the animals. Sense organs capable to respond to light stimulus are photoreceptors. Osphradium in molluscs can test the quality of water and it is considered as a chemoreceptor. This suggests some sort of sense of smell are present in them. The pulmonate molluscs move towards the food, even after removal of both the eyes by surgery. Olfactory receptors in moths and ants are quite powerful. The antennae and palpi in insects bear receptors for smell. Not much is known about sense of smell in the invertebrates.
The taste receptors in flies are specific for sugar, water and salts. Receptors for taste and touch are present in these structures. The mouth parts and terminal segment of butterflies and moths bear hairs and pegs. These sense organs are capable to differentiate between chemicals and thereby help in controlling the activities of animals in relation to its surroundings, in the selection of food, in social behaviour, in defence from predators, in selection of mate and others. Receptors for smell and taste are chemo- receptors. They can only perceive the disturbances in the medium - air, water or solids. Hearing, in true sense of the term, is absent in invertebrates. Nerve endings of sensory neurons are present in the cuticle covering it. It consists of parallel slits of varying length in the cuticle, resembling the strings of a lyre. The sensory neurons attached to the apical bodies, the scolopodia are bipolar. The type known as tymponal organ, the most complex and efficient organ of sound perception is found in moths. They differ in structures in different arthropods. Frequency of sound waves below 20 hertz is called vibration. They react with the sound waves and are located on the body surface.
The hairs can perceive low frequency vibrations. Three types of phonoreceptors have been identified in arthropods: Phonoreceptors or sense of hearing can detect pressure waves caused by mechanical disturbances at some distance from the body. A pair of halteres, the modified hind wings in flies and mosquitoes take care of maintaining stability by controlling lift, rotation and landing in their flight. With the change in the body position, the particles exert pressure on certain sensory cells and this signals the animal to correct its position with respect to the gravity.
A statocyst is usually an oval sac, may or may not be in communication with the exterior and lined internally by a sensory membrane bearing processes or hairs.Ī single calcareous particle or statolith or a number of small particles, statoconia (Fig. statas = standing + kystis = sac) are ‘organs of balance in arthropods and molluscs.Ī statocyst is usually an oval sac, may or may not be in communication with the exterior and lined internally by a sensory membrane bearing pro- arthropods and molluscs. 14.2.), the marginal sense organs, ensure horizontal swimming in cnidarians. Equilibrium receptors respond to loss of balance and help the animal to correct it. Maintenance of body equilibrium is essential for the normal activities in an animal. and located on the stretchable area of the cuticle.Īction of wings in flight is controlled by proprioreceptor cells present at the base of the halteres. In insects they are modified cuticular structures in the form of hair, plate, etc. In the invertebrates they are concerned with transmitting postural information and are present in crustacea, and insects. The proprioreceptors are tonic receptors, and register stress changes in the exoskeleton.