I04 FIRST LESSONS IN ZOOLOGY 



ort^ans arc found wliich from their structure seem to be 

 sense-oryans althoutjli we are by no means sure what 

 kind of sense the}' serve. 



In some of tlie lower animals, as the polj'ps, 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 ma\' develop are called primitive or generalized 

 sense-organs. 



The tactile sense or sense of touch is the simplest 

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

 simplest organs. The special organs arc usualh" simple 

 liairs or papilla.- connecting with a nerve. They ma)' be 

 distributed pretty evenl)' over most of the body or may 

 be mainly concentrated upon certain parts in crowded 

 groups. Many of the lo^\■er animals have projecting 

 parts, like the feeling tentacles of many marine inverte- 

 brates, or the anteniiic (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 in\-ertebrates, or arc little sac-like bodies of connec- 

 ti\'e tissue in which the end of a nerve is curiously folded 

 ant! convoluted. These little touch-corpuscles (fig. 70) 

 lie in the cell la)'er of the skin, co\'erec^ o^'er thinl)' by 

 the cuticle. Srjmetimcs the)' are simply free, branched 

 nerve-endings in the sl<in. In either case they are 

 especiall)' abundant in those parts of the bod)' which 

 can be best used fur feeling. In man the finger-tips are 

 thus special 1)' supplied, in cei'tain tailed monkeys the tip 

 of the tail, and in hogs the end of the snout. 



The taste oi'gans ai'e much lilsc the tactile organs ex- 

 cept that the s[)ecial taste cell must be exposetl, so that 

 small particles of the substance to be tasted can come 



