The Theory of Evolution 63 



of higher vertebrates, and the number that open to the ex- 

 terior are variable ; but the number of gill-openings that are 

 present in the adults of lower vertebrates is also variable. 

 No one who has studied the method of development of the 

 gill-slits in the lower and higher vertebrates will doubt for a 

 moment that some kind of relation must subsist between 

 these structures. 



In the lowest adult form of the vertebrates, amphioxus, 

 the gill-system is used largely as a sieve for procuring food, 

 partly also, perhaps, for respiration. In the sharks, bony 

 fishes, and lower amphibians, water is taken in through the 

 mouth, and passes through the gill-slits to the exterior. 

 As it goes through the slits it passes over the gills, that 

 stand like fringes on the sides of the slits. The blood that 

 passes in large quantities through the gills is aerated in 

 this way. In the embryos of the higher vertebrates the 

 gill-slits may appear even before the mouth has opened, 

 but in no case is there a passage of water through the 

 gill-slits, nor is the blood aerated in the gill-region, although 

 it passes through this part on its way from the heart to 

 the dorsal side of the digestive tract. It is quite certain 

 that the gill-system of the embryo performs no respiratory 

 function. 1 



In the higher amphibians, the frogs for example, we find 

 an interesting transition. The young embryo, when it 

 emerges from the egg-membranes, bears three pairs of 

 external gills that project from the gill-arches into the sur- 

 rounding water. Later these are absorbed, and a new 

 system of internal gills, like those of fishes, develops on 

 the gill-arches. These are used throughout the tadpole 

 stage for respiratory purposes. When the tadpole is about 

 to leave the water to become a frog, the internal gills are 



1 This statement is not intended to prejudice the question as to whether the 

 presence of the gill-slits and arches may be essential to the formation of other 

 organs. 



