ON THE ORIGIN OF VERTEBRATES. 267 



Pantopoda and the Tardigrada must probably be regarded as 

 smaller independent branches of the Arthropodan stock." To 

 these three great series of development of the Arthropodan 

 stock the evidence of Ammocwtes shows that a fourth must be 

 added, which, starting also from the Protostraca, and closely 

 connected with the second Pakeostracan branch, leads through 

 the Cephalaspida to the great kingdom of the Vertebrata. 

 Such a direct linking of the earliest vertebrates with the 

 Annelida through the Protostraca is of the utmost importance, 

 as will be shown later in the explanation of the origin of the 

 vertebrate coelom and urinary apparatus. 



