116 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 Paleostraca (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 Ammoccetes shows that a fourth must be added, 
which, starting also from the Protostraca, and closely connected with 
the second, paleostracan branch, leads through the Cephalaspide 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. 
