II 



THE COMING OF A BACKBONE 



PEERING backward into the dawn of geological history, 

 where even the palaeontologist can help us but little, 

 we see dimly a vast ocean peopled with a swarm of 

 worms and coelenterates, as well as all sorts of still lower 

 life. The age of pacifists, of universal flabbiness and peace, is 

 passing fast. It was high time to begin the development of 

 some sort of skeleton. Three different types of skeleton were 

 possible. Different descendants of worms experimented on 

 these three lines and thus became ancestors of the highest 

 groups of animals: vertebrates, arthropods and mollusks. 

 These three ancestral, pioneer worms parted company ages 

 ago, and all unconsciously went out into the world to seek 

 their fortunes. The vertebrate ancestor developed an internal 

 locomotive skeleton which was to culminate in a backbone. 

 The arthropods are represented by insects, spiders and crabs. 

 Their ancestor secreted an external horny skeleton, chiefly 

 for locomotion, partly for protection. 



The primitive molluscan ancestor was apparently an un- 

 segmented worm crawling slowly and lazily over the bottom 

 where small food was abundant. Here, where life teems, 

 some sort of protective skeleton seems almost the rule. There 

 are shelled protozoa, coral-animals and sessile well protected 

 worms. The skin takes easily and naturally to producing 

 such a skeleton, and very likely it developed rapidly. He 

 crawled slowly on the bottom, and his mode of life gradually 

 molded his whole structure. 



We will first glance at the clam, the extreme logical con- 

 clusion and in a way the consummate flower of molluscan 



19 



