THE RECAPITULATION THEORY 177 
At a later stage most of the gill-pouches have been obliterated, but one 
is retained and converted into the Eustachian canal, which connects 
the throat with the middle ear, inside of the ear-drum. Similarly, the 
embryological evidence shows that the lungs of air-breathers have been 
derived from the swim-bladder of fishes, a conclusion which had 
already been reached by comparative anatomy, for in a remarkable 
Fic. 39.—Embryos ini corresponding stage of development of shark (A), 
fowl (B), and man eh &; ay slits. -(From Scott.) 
group, known as the eatin or lung-fishes, the air-bladder is utilized 
for purposes of respiration. 
It has been objected that, while ebay may prove relation- 
ship within a single type, it fails to demonstrate any connection 
between different types, but this is not altogether true. The Tuni- 
cata, a curious group of marine animals once referred to the Mollusca, 
are shown by their ontogeny to be related to the vertebrates and the 
-same is.true of certain marine worms (Balanoglossus). Indeed, most 
modern zodlogists have adopted a scheme of classification, in which 
