72 VERTEBRATE ZOOLOGY 



THE AMPHIOXUS THEORY OF VERTEBRATE ORIGIN 



Amphioxus, called by someone the "chordate Adam and Eve," 

 bears many evidences of being a very antique type. Its cosmopolitan 

 distribution, the essential uniformity of all the different species, and 

 its almost diagrammatic simplicity argue for its status as a truly primi- 

 tive creature. Even though it appears to be somewhat degenerate as 

 to its head parts and though it is suspected by some authors of being 

 pxdogenetic, there is much to show that its fundamental structures 

 are nearly prototypic. 



The habits of Amphioxus show an interesting combination of two 

 types of life, the sedentary and the free-swimming. While they are 

 capable of rapid and vigorous darting movements in the water, they 

 spend much of the time as sedentary creatures, living in burrows in 

 the sand with only the oral end protruding (Fig. 9) . This life along the 

 sandy shores requires just exactly this double adaptation, since it is 

 in this region that we would find the most effective tidal currents. 

 The lancelets are evidently able to swim rapidly against the tidal 

 currents and to change their locations as often as the state of the tide 

 demands. They also burrow into the sand by vigorous undulating 

 movements and, when once buried, they remain for long periods 

 quietly feeding, as shown in Fig. 9, by means of their unique food- 

 concentrating mechanism, a complex of structures that is highly 

 adapted for sedentary life and ill adapted for active free-swimming 

 life. The lancelet then has a combination of two opposed tendencies, 

 that of the quiet sedentary animal, and that of the free-living rapid- 

 swimming animal. Specialization might readily be carried out in 

 either direction. 



If specialization followed the lines of an increasing fixity of position, 

 we might easily conceive of a type of lancelet that became fixed while 

 still a larva . The functional stimulus for further development of the 

 locomotor musculature and the notchord would be wanting and the 

 resultant creature would consist largely of a food-concentrating and 

 digestive body without any of the organs essential for active life. 

 This we believe is just what happened. Some of the ancestral lance- 

 lets migrating into deeper waters, where the lack of tidal currents 

 would no longer stimulate the swimming about of the larvae and 

 young, would remain stationary throughout life, would not even be 

 able to make burrows, but would fix themselves to rocks, etc., at the 



