EMBRYONIC AND LARVAL HISTORIES 73 



in the adult structure of Balanoglossus that could 

 possibly lead anyone to connect it with the adult 

 Echinoderm, and yet the larval forms of these two 

 totally different kinds of animals are for all purposes 

 identical. We cannot avoid the conclusion that this 

 identity proves that Balanoglossus and the Echino- 

 derms at some remote epoch possessed a common 

 ancestor with this larval form, which has been pre- 

 served during the immense period in which the 

 divergent evolution of the two adult types has pro- 

 ceeded. The conclusion surely involves a very great 

 antiquity for this larval type, which thus is seen to be 

 a link with the past of a very persistent kind. 



We have touched so far only upon certain ancient 

 larval forms, but we might have illustrated the anti- 

 quity of many developmental phases by reference to 

 the embryonic stages of many animals, passed in a 

 quiescent state within the egg membranes. Space will 

 only permit us to refer to one such instance, namely 

 the persistent development of gill-like structures 

 in the embryos of all Vertebrate animals, even 

 in those Vertebrate animals, such as birds, reptiles 

 and mammals, which never spend any part of their 

 existence in the water breathing by means of gills. 



The whole structure of the head in these animals 

 is fundamentally affected by the fact that in the 

 young embryo at least three and usually more gill- 

 clefts are formed with their associated supporting 

 structures, blood-vessels, nerves and muscles, in which 



