Tke Story of Evolution 73 



6 



Great Acquisitions 



In animals like sea-anemones and jellyfishes the general 

 symmetry of the body is radial ; that is to say, there is no right or 

 left, and the body might be halved along many planes. It is a 

 kind of symmetry well suited for sedentary or for drifting life. 

 But worms began the profitable habit of moving with one end of 

 the body always in front, and from worms to man the great ma- 

 jority of animals have bilateral symmetry. They have a right 

 and a left side, and there is only one cut that halves the body. 

 This kind of symmetry is suited for a more strenuous life than 

 radial animals show; it is suited for pursuing food, for avoiding 

 enemies, for chasing mates. And with the establishment of 

 bilateral symmetry must be associated the establishment of head- 

 brains, the beginning of which is to be found in some simple 

 worm-types. 



Among the other great acquisitions gradually evolved we 

 may notice: a well-developed head with sense-organs, the es- 

 tablishment of large internal surfaces such as the digestive and 

 absorptive wall of the food-canal, the origin of quickly contract- 

 ing striped muscle and of muscular appendages, the formation of 

 blood as a distributing medium throughout the body, from which 

 all the parts take what they need and to which they also 

 contribute. 



Another very important acquisition, almost confined (so far 

 as is known) to backboned animals, was the evolution of what 

 are called glands of internal secretion, such as the thyroid and the 

 supra-renal. These manufacture subtle chemical substances 

 which are distributed by the blood throughout the body, and have 

 a manifold influence in regulating and harmonising the vital 

 processes. Some of these chemical messengers are called hor- 

 mones, which stimulate organs and tissues to greater activity; 

 others are called chalones, which put on a brake. Some regulate 



