THE SPECIAL SENSES 373 



body, without organs, and yet with its capacity for per- 

 forming the necessary life processes; there are no special 

 senses except one (perhaps two). The Amc&ba can feel. 

 It possesses the tactile sense. And there are no special 

 sense organs except one, which is the whole of the outer 

 surface of the hody. If the Amcefia he touched with a 

 fine point it feels the touch, for the soft viscous proto- 

 plasm of its body flows slowly away from the foreign ob- 

 ject. The sense of feeling or touch, the tactile sense, is 

 the simplest or most primitive of the special senses, and 

 the simplest, most primitive organ of special sense is the 

 outer surface or skin of the body. Among those simple 

 animals that possess the simplest organs of hearing and 

 perceiving light, we shall find these organs to be simply 

 specialized parts of the skin or outer cell layer of the 

 body, and it is a fact that all the special sense organs of 

 all animals are derived or developed from the outer cell 

 layer, ectoblast, of the embryo. This is true also of the 

 whole nervous system, the brain and spinal cord of the 

 vertebrates, and the ganglia and nerve commissures of 

 the invertebrates. And while in the higher animals the 

 nervous system lies underneath the surface of the body, 

 in many of the lower, many-celled animals all the ganglia 

 and nerves, all of the nervous system, lie on the outer 

 surface of the body, being simply a specialized part of 

 the skin. 



292. The sense of touch. In some of the lower, many- 

 celled animals, as among the polyps, there are on the skin 

 certain sense cells, either isolated or in small groups, which 

 seem to be stimulated not alone by the touching of foreign 

 substances, but also by warmth and light. They are not 

 limited to a single special sense. They are the primitive 

 or generalized organs of special sense, and can develop into 

 specialized organs for any one of the special senses. 



The simplest and most widespread of these special 

 senses with, as a whole, the simplest organs, is the tactile 



