PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 525 



of several pairs of oral arches to form the unpaired jaws, and some 

 of the adjacent organs of the adult (Fig. 8). 







Fig. 7. ^-Z). Development of the jaws of the sturgeon. After Sa- 

 lensky. E, head of Bdellostoma, a primitive vertebrate, showing three 

 pairs of oral appendages similar to those in the frog embryo. F , Bothrio- 

 lepis, an ancient, extinct animal of vertebrate affinities, with mouth parts 

 intermediate in character between those of arthropods and vertebrates. 'From 

 Patten, " The Evolution of the Vertebrates and their Kin." 



The conditions that led to the opening of the ingrowing gill 

 chambers into the outgrowing enteric diverticula are not apparent. 



