Transformations of Organs 73 



monkeys and man, where its place is taken by the 

 upper eyelid, which is much more mobile than in 

 ordinary mammals. Behind the eye of the skate there 

 is a large hole, opening into the back of the mouth. It 

 is called the spiracle and is the first of the gill-clefts. 

 It serves for the entrance of the water which washes 

 the gills and passes out by the five pairs of ordinary 

 gill-clefts on the ventral surface. The spiracle is 

 therefore of much use; but when we look into it we 

 see a little comb-like structure which is plainly a ves- 

 tigial gill. The minute teeth of the comb correspond 

 to the filaments of a gill, but they are too small to be 

 of any use in respiration. The comb is clearly a ves- 

 tigial gill. It may be noted that the spiracle of the 

 skate and other Gristly Fishes or Selachians becomes 

 the Eustachian tube in Amphibians, Reptiles, Birds, 

 and Mammals — a tube running from the outer ear- 

 passage past the inner ear to the back of the mouth. 

 This is a good example of change of function in the 

 course of evolution. 



Another point of interest is that many Gristly 

 Fishes have below the vestigial gill a patch of peculiar 

 cavernous tissue, which is said to be of importance 

 in the destruction of worn-out blood-corpuscles and 

 in the making of new ones to take their place. We see, 

 then, that the spiracle itself is not vestigial: it is a 

 cleft for the entrance of water ; and the tissue below 

 the comb is not vestigial : it has hemolytic and hemo- 

 poietic functions; what is vestigial is the dwindling 

 comb. True vestigial structures are minute traces of 

 what they once were and have dwindled out of use. 



