212 THE SENSES OF ANIMALS 



TENTACULAR 

 CIRRI 



Fig. 17. The head end of a marine worm {Nereis virens) to show the eight 

 "feelers" (tentacular cirri) beside the four eyes. 



are richly provided with touch-receptors, but they also bear 

 receptors for other senses such as that of smell or taste. In 

 addition most of these animals bear receptors on the cuticle 

 that covers the armour, such as the sensilla already men- 

 tioned, which occur on the bodies of insects. Some of these 

 are certainly touch-receptors, but others are probably chemo- 

 receptors, or sound- and vibration-receptors. 



The sense of touch has considerable powers of discrimina- 

 tion when many of the receptors are stimulated at the same 

 time. If only a few are stimulated we have the sensation of 

 something the nature of which we cannot determine touch- 

 ing us; if the touch is light it merely "makes the place 

 tickle". If a larger area of skin comes in contact with the 

 object, so that many receptors respond, we know whether 

 the object is rough or smooth, sticky, soft or hard. It is 

 peculiar that some of the sensations produced are pleasant 

 to us and others are not ; we dislike the feel of sticky glue 

 or rough sandpaper, but enjoy the feel of sleek fur, cool 

 marble or polished jade. Now technology has provided us 

 with a new sensation ; the frictionless plastics that are now 

 being introduced for the bearings of machinery are outside 

 our normal experience — if you hold a piece in your hand 

 you are aware of its presence through its weight, yet you 



