Il6 THE ORIGIN OF VERTEBRATES 



together to form a single lens, instead of being collected in groups of 

 four to form a series of crystalline cones. 



To sum up : The study of the vertebrate eyes, both median and 

 lateral, leads to most important conclusions as to the origin of the 

 vertebrates, for it shows clearly that whereas, as pointed out in this 

 and subsequent chapters, their ancestors possessed distinct arachnid 

 characteristics, yet that they cannot have been specialized arachnids, 

 such as our present-day forms, but rather they were of a primitive 

 arachnid type, with distinct crustacean characteristics : animals 

 that were both crustacean and arachnid, but not yet specialized in 

 either direction : animals, in fact, of precisely the kind which 

 swarmed in the seas at the time when the vertebrates first made their 

 appearance. In the opinion of the present day, the ancestral forms 

 of the Crustacea, which were directly derived from the Annelida, 

 may be classed as an hypothetical group the Protostraca, the nearest 

 approach to which is a primitive Phyllopod. 



" Starting from the Protostraca," say Korschelt and Heider, 

 " according to the present condition of our knowledge, we may, as 

 has been already remarked, assume three great series of development 

 of the Arthropodan stock, by the side of which a number of smaller 

 independent branches have been retained. One of these series leads 

 through the hypothetical primitive Phyllopod to the Crustacea ; the 

 second through the Pakeostraca (Trilobita, Gigantostraca, Xiphosura) 

 to the Arachnida ; the third through forms resembling Peripatus to 

 the Myriapoda and the Insecta. The Pantapoda 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 Ammocoetes shows that a fourth must be added, 

 which, starting also from the Protostraca, and closely connected with 

 the second, palffiostracan branch, leads through the Cephalaspidae 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 ccelom and urinary apparatus. 



