l62 



THE ORIGIN AND EVOLUTION OF LIFE 



types of unarmored fishes. The double-pointed, fusiform body, 

 in which the segmented propelHng muscles are external and a 

 stiffening notochord is central, is the fish prototype, which 



MYOMERES 

 iscfe segments) 





SPIKAi KEHV[ CORD 



^//>^ 



w///AM^ym^y/^'\^ 



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OIGEST/te TRUCr 



more or less clearly 

 survives in the exist- 

 ing lancelets (Aniphi- 

 oxus) and in the lar- 

 val stages of the de- 

 generate ascidians. 

 These animals also 

 furnish numerous 

 embryonic and lar- 

 val proofs of de- 

 scent from nobler 

 types. 



Following the 

 pro-fishes of Ordovi- 

 cian time, the great group of true fishes begins its form evolu- 

 tion with (^4) active, free-swimming, double-pointed types of 

 fusiform shape, adapted to rapid motion through the water 

 and to predaceous habits in pursuit of swift-moving prey. 



I'lG. 43. TuK Existing Lanceleis [Aiiipliioxns). 



Fusiform protochordates living in the littoral zone of 

 the ocean shores, sole survivors of an extremely 

 ancient stage of chordate (pro-vertebrate) evolution. 

 The body is fusiform or doubly pointed, hence the 

 name Amphioxiis. It is stiffened by the continuous 

 central axis (chorda, notochord). All the other or- 

 gans are more or less sharply segmented. After Willey. 



