296 ANIMAL LIFE 
one-celled body, without organs, and yet with its capacity 
for performing the necessary life processes, there are no 
special senses except one (perhaps two). The Ameba 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 body. If the Ameba be 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. 
119. 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 
