TISSUE-DIFFERENTIATION 107 



screw and largely increasing the absorptive surface. 

 In mammals the whole inner surface of the intestine 

 is lined with innumerable finger-shaped processes 

 called villi that project into the intestinal cavity and 

 enormously increase the absorptive area. 



Sensory Organs. Since food must usually be 

 captured before being eaten, we find that in the great 

 majority of animals (the "higher" ones particularly) 

 all the sense organs available, whether of seeing or 

 touching, smelling or tasting, are concentrated in 

 close proximity to the mouth. This specialization of 

 organs in one portion of the body has brought about 

 a sharp differentiation of that part from the rest of 

 the body, a cephalization or " head-specialization." 

 Sensory organs in the beginning were probably 

 diffusely scattered over the body as they are now in 

 the simplest types, and the concentration of this 

 system in one locality, i.e. the head, has made neces- 

 sary a means of communication between such centers 

 and more remote parts. Thus we find a system of 

 nerves connecting the sensory centers with other 

 parts of the body, particularly the muscles, and 

 affording paths for stimulating "impulses" which 

 result in coordinating movements and thus unifying 

 action. 



Concentrations of nervous elements into centers of 

 this sort are termed ganglia (singular, ganglion). 

 The ganglia are larger and more complex in the region 

 where the majority of sensory impulses arise, that is, 

 the head, and in higher forms of animal life these 



