318 WILLIAM PATTEN. 



In failing to add materially to what the anatomy and 

 embryology of Vertebrates themselves can demonstrate, the 

 Annelid theory not only is sterile^ but is likely to remain so ; 

 because unspecialised segments being characteristic of Annelids^ 

 it cannot hope to elucidate that profound specialisation of the 

 Vertebrate head which it is the goal of Vertebrate morphology 

 to expound. Moreover, since Vertebrate morphology itself 

 reflects as an ancestral image only the dim outlines of a 

 segmented animal — but still not less a Vertebrate than any 

 now living, — it is clear that the problem must be solved^ if at 

 all, by the discovery of some form in which the specialisation 

 of the Vertebrate head is already foreshadowed. 



Since of all Invertebrates, concentration and specialisation 

 of head segments is greatest in the Arachnids, it is in these, on 

 a priori grounds, that we should expect to find traces of the 

 characteristic features of the Vertebrate head. Finding from 

 time to time confirmation of this preconceived idea as the 

 unexpected complexity of the Arachnid cephalothorax revealed 

 itself, I now feel justified in formulating a theory that Verte- 

 brates are derived from Arachnids. 



I have presented the facts as they appear to me, and have 

 hazarded an interpretation of them ; not, however, without 

 a lively sense of the difficulties of the task, certainly not 

 without the conviction that I may have fallen into errors which 

 greater experience and a better knowledge of the intricacies 

 of Vertebrate anatomy might have avoided. 



In the following preliminary sketch of the structure of 

 Limulus, and especially of the Scorpion, I shall attempt to 

 prove — (1) That in the Scorpion the cephalothoracic neuro- 

 meres, nerves, sense-organs, and mesoblastic somites present, in 

 a general way, not only the same specialisation and the same 

 numerical arrangement in groups, but also the same difference 

 as a whole from the body-segments, as do the corresponding 

 parts in the Vertebrate head ; (2) that the Arachnid cartila- 

 ginous sternum represents the primordial cranium of Verte- 

 brates ; (3) that in the Trilobites and Merostomata the internal 

 structure of the cephalothorax resembles in some respects that 



