THE ORIGIN OF MAN 37 



animals, with a distinct head and tail, and breathe 

 through the anterior region of the intestine (in- 

 testinal breathers). In this group in Balano- 

 glossus the rudiments of the future gills, the 

 breathing organs of fishes, are seen. Further- 

 more, the main nerve-cord is situated dorsally 

 and the intestinal tract ventrally, and these are 

 the fundamental structures around which the ver- 

 tebrate organization has been evolved. 



Balanoglossus burrows in sand and mud in al- 

 most all seas. The anterior end is formed of 

 the burrowing proboscis, beneath which on the 

 ventral side is the mouth, followed behind by the 

 collar, then the region of gill slits, and finally a 

 long, soft, slightly coiled portion. There is a 

 dorsal nerve-cord, often tubular, and like a typ- 

 ical spinal cord. The dorsal region of the 

 pharynx is respiratory and the water passes 

 through the gill slits, which open dorsally on the 

 exterior. 



The next step is seen among the sea-squirts 



