7 o THE ANIMALS AND MAN 



their structure seem to be sense-organs although we are by 

 no means sure what kind of sense they serve. 



In some of the lower animals, as the polyps, there are 

 on the skin certain sense-cells, either isolated or in small 

 groups that are not limited to a single special sense. They 

 seem to be stimulated not alone by the touching of foreign 

 substances, but also by warmth and light. These simple 

 sense-cells from which the more complex or special ones 

 may develop are called primitive or generalized sense- 

 organs. 



The tactile sense or sense of touch is the simplest and 

 most wide-spread of the special senses, with the simplest 

 organs. The special organs are usually simple hairs or 

 papillae connecting with a nerve. They may be distributed 

 pretty evenly over most of the body or may be mainly con- 

 centrated upon certain parts in crowded groups. Many 

 of the lower animals have projecting parts, like the feeling 

 tentacles of many marine invertebrates, or the antennae 

 (feelers) of crabs and insects, which are the special seat of 



the tactile organs. Among the vertebrates 

 the tactile organs are either like those 

 of the invertebrates, or are little sac-like 

 bodies of connective tissue in which the 

 end of a nerve is curiously folded and 

 convoluted. These little touch-corpuscles 

 (fig. 27) lie in the cell layer of the skin, 

 covered over thinly by the cuticle. Some- 

 FIG. 27. Tactile cor- t j mes tnev are s i m piy f ree branched nerve- 



puscle of the skin / i . i 



of man; n, nerve, endings in the skin. In either case they 

 (Greatly magni- ar e especially abundant in those parts of 



fied; after Kolli- the body wh j ch can be begt used for fed _ 



ing. In man the finger-tips are thus es- 

 pecially supplied, in certain tailed monkeys the tip of the 

 tail, and in hogs the end of the snout. 

 The taste organs are much like the tactile organs except 



