532 THE CHANGING WORLD 



bilateral type, making possible the revolutionary head end. In this 

 device a suitable home was furnished for the brain when it should 

 appear, with exploratory sense organs near by, placed handily to best 

 advantage for receiving impressions from the immediate surroundings. 

 Following this by the device of metamerism, which is a division of 

 the body into segments, primitive fiatworms took on annelid charac- 

 teristics, thus acquiring flexibility after the manner of a train of cars, 

 and also gaining survival insurance against accidental loss of parts. 



Locomotor legs soon came in to lift the long crawling body off the 

 ground, thereby much lessening friction in traveling about. Legs 

 developed joints, adding the mechanical advantage of levers. Various 

 experiments in the number of legs were tried out with a result of 

 increasing efficiency. Myriapods and centipedes had too many. 

 Crustaceans began a reduction. Spiders and their allies brought 

 the number down to eight, while the great group of insects finally 

 settled upon six legs as the prevailing fashion. It remained for verte- 

 brates to get along at first with only four legs, one under each corner 

 of a horizontal body, like the legs of a table. Eventually in the case 

 of birds and man, and some other vertebrates, only a single pair of 

 legs remains at the end of an upright vertical body. It is likely that 

 evolution has reached its limit with reduction to one pair of locomotor 

 legs, since a one-legged animal would obviously be at a disadvantage. 

 However, with these legs we have run ahead of our story. 



When skeletal parts for muscular attachment first developed, 

 making locomotor legs workable, the skeleton was an exoskeleton on 

 the outside of the body. Being secreted by the underlying tissues, 

 and consequently a dead structure unyielding and hampering to the 

 enlarging body within, it is soon outgrown and has to be discarded, 

 frequently at considerable peril and physiological expense, to allow 

 for future growth. It was a great day for us when our ancestors 

 put their skeletons inside the body and became vertebrates. The 

 vertebrate endoskeleton can remain alive and continue growing, and 

 can thus keep up with the demands of increasing body size. 



Many other evolutionary experiments were tried out in the course 

 of time, resulting in the establishment of the great major phyla of the 

 animal kingdom, but it was the vertebrate idea which finally forged 

 ahead upward toward man. 



The fishes served a long apprenticeship as the lowest vertebrates, 

 principally in the waters of the great oceans that cover most of the 

 globe, until eventually there developed those adventurous pioneers, 



