ON THE ORIGIN OF VilRTEBRATES PROM ARACHNIDS. 317 



On the Origin of Vertebrates from Arachnids.^ 



By 



liVilliam Patten, Pb.D., 



Professor of Biology in the University of North Dakota, Grand Forks. 



With Plates XXIII and XXIV. 



" In the growth of each science, not only is correct observation needful for 

 the formation of true theory, but true theory is needful as a preliminary to 

 correct observation." — H. Spencer. 



^ The " Annelid theory,'^ after fifteen years of dexterous 

 modellings is now as far as ever either from fitting the facts of 

 Vertebrate structure, or from shedding any direct light on the 

 great problem of the origin of Vertebrates. It certainly is not 

 without significance that^ of all those who with willing eyes 

 and minds have grappled with the Annelid theory, not one has 

 discovered a distinctively Annelid feature in Vertebrates : 

 mesoblastic somites, nephridia, segmental appendages, and seg- 

 mental sense-organs are found in nearly all segmented 

 animals. 



1 I include in the Arachnida the Spiders, Scorpions, Limulus, Trilobites, 

 and Merostomata. 



" Most of my observations on Acilius, Scorpio, and Limulus 

 were made in the Lake Laboratory, Milwaukee, Wis. I am greatly indebted 

 to the founder of that institution, Mr. E. P. AUis, for generously placing at 

 my disposal the excellent facilities for research which his laboratory affords. 



As a full description of my observations could not be published without 

 considerable delay, it seemed advisable to present my theoretical conclusions 

 first, at the same time giving a short account of those facts bearing directly on 

 the subject-matter. 



A few simple diagrams have been introduced to make the text more 

 intelligible. These are throughout referred to as Figs. 1, 2, 3, &c. The 

 reference to figures in the two plates is always indicated by the addition of 

 the letters PI. XXIII or PI. XXIV. 



VOL. XXXI, PART III. — NEW SER. Y 



