PLANT AND ANIMAL PROGRESSION 49 



symmetry to a plan of mirror images along a median axis 

 (so-called bilateral symmetry). Nature found this plan very 

 good, and it is carried through to man who is, indeed, very 

 handsome bilaterally. Bilateral symmetry placed one end of 

 the animal in a forward position and, more than anything 

 else, was responsible for the beginning of a brain. In the 

 flatworms it is a ganglion with longitudinal and lateral con- 

 necting nerves. Roundworms take over all the gadgets of 

 the fiatworm but establish a posterior opening to the diges- 

 tive system; and we have the "tube-within-a-tube," bilateral 

 plan of the higher animals, man included. Annelid worms 

 break up the tube in segments and add locomotor append- 

 ages based on the physics of the lever, namely, two rows 

 of bristles very cleverly manipulated. A heart and circula- 

 tory system is added and the primitive beginnings of an 

 excretory system appear. The nervous system is improved. 

 This animal, of which the earthworm is a slightly degener- 

 ate representative, already represents basically the systemic 

 plan of higher forms. "Man," someone has said, "is a worm 

 with accessories." 



The Arthropoda (jointed foot), of which the insect is a 

 fabulously successful representative, are glorified worms. 

 They have put joints in the locomotor appendages; have 

 fused the segments into a head, thorax, and abdomen; have 

 redistributed the now rather complete list of internal organs 

 for greater efficiency; and have added to the over-all and 

 proportionate size of the nervous system. Some of this 

 group have developed incredible powers of instinctive be- 

 havior, a situation that will be examined later. 



Somewhere above the level of flatworms there appears a 

 strange group called the Echinodermata, the spiny-skinned 

 animals, of which the starfish is an example. These forms, 

 which may have originated from ancestral types close to 

 those of man, represent nature's inexhaustible genius for 

 invention. Although they are connected with other inver- 

 tebrate groups through their bilateral larval stages, they are 



