526 PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 



although such an opening seems to have taken place at several differ- 

 ent times in the evolution of the arachnid stock. 



Fig. 8. Human embryos showing the three pairs of oral arches that 

 help to form the face and jaws. From Patten, "A Problem in Evolution." 



But the facts we wish to emphasize here are that the communica- 

 tion in vertebrates of the gill chambers with the alimentary canal has 

 two very different effects: (i) it greatly increases the respiratory 

 power by directing the respiratory current through the gills in a 

 constant direction, that is, in at one side and out the other, instead of 

 in and out through the same opening; and (2) it at once makes 

 it impracticable to carry on digestive action at any point in front of 

 an open visceral cleft. 



The thyroid gland is doubtless the remnant of the prebranchial 

 digestive glands^ modified, or temporarily thrown out of commission, 

 by the opening up, in this manner, of the visceral clefts (Fig. 9). 



The corresponding organs of the arachnids (scorpion) are vol- 

 uminous thoracic, or prebranchial, digestive glands, which in their 

 grosser morphological relations, and in their histological structure, 

 strongly resemble the thyroids of vertebrates. 



