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. 
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 gillarches. 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. 
