SENSORY ORGANS AND RECEPTION 335 



sources. Sound waves or high-frequency vibrations are detected by special 

 phonoreceptors. Many animals possess tension receptors, responsive to 

 distortion by stretch. Finally, special mechanoreceptors, the gravity and 

 equilibrium receptors, permit the animal to orient in space. 



Sensitivity to Touch and Low-Frequency Vibrations 



The importance of tactile stimuli in the behaviour of animals requires 

 no emphasis. Benthic species are continuously being exposed to contact 

 stimuli of various kinds, free-swimming species respond to contact with 

 solid objects, and many animals are sensitive in various degrees to distant 

 vibrations. Tactile receptors may act as external proprioceptors, providing 

 information about spatial position and movement of parts of the body 

 and in this regard they act in conjunction with internal proprioceptors 

 and equilibrium receptors. 



Tactile Sensitivity. The simplest type of tactile receptors are free 

 nerve endings lying in the skin. These endings may show generalized 

 irritability, being sensitive to a variety of noxious stimuli, mechanical, 

 thermal and chemical. More primitive animals, such as flatworms, anne- 

 lids, enteropneusts, etc., possess individual sensory neurones scattered 

 through the epidermis (Fig. 8.2). These may take the form of fusiform 

 elements bearing sensory hairs distally, and giving rise to nerve fibres 

 which extend toward the central nervous system (c.n.s.) (94). Crustacea 

 have sensory hairs distributed over the surface of the body, especially at 

 joints of the appendages. Each hair is served by a sensory neurone at its 

 base. Free nerve endings are abundant in the skin offish, but encapsulated 

 endings, such as occur in higher vertebrates, are rare (160). 



Local tactile stimulation produces diverse responses. A gentle stimulus 

 evokes movement in a restricted area, e.g. in a single palp or antenna. 

 Under strong stimulation sessile and sedentary animals display withdrawal 

 reflexes involving strong contraction of the body musculature, e.g. sea 

 anemones, polychaetes, holothurians, phoronids, etc. Other reflexes 

 induced by tactile stimulation are luminescence, colour changes, display 

 of protective armament, operation of poisonous devices, etc. 



Contact stimuli produce orientation reflexes of various kinds. Benthic 

 animals such as polychaetes, which crawl over the substratum, tend to come 

 to rest in crevices or at angles between surfaces. In the absence of other 

 suitable contact stimuli they may bunch together. This type of orientation 

 reflex is termed thigmotaxis. When an animal is resting on the substratum 

 it experiences continuous asymmetrical stimulation of its tactile receptors. 

 In the absence of ventral surface-stimulation, righting reflexes are initiated, 

 e.g. in gastropods, starfishes, etc. Another kind of orientation reflex is 

 rheotaxis, or orientation to water currents. Visual cues are often pre- 

 dominant in orientation, as in lobster and fish, but blind fish can orient 

 themselves in a current when they are resting on the bottom, through 

 frictional stimulation of contact receptors. 



Regional differences in tangosensitivity have been studied in various 



