ON THE STRUCTURE OE THE PTERASPIM 

 AND CEPHALASPID/E. 



I. The fact which first suggested to me the possibility of a 

 genetic relationship between the Yertebrata and Arthropoda was 

 the similarity in structure and development between the median 

 eyes of arthropods and the pineal eyes of vertebrates. 



To test this idea a careful study of the brain, sense organs, 

 cranial nerves, nephridia and skcletogenous structures was made, 

 the results of which showed so clearly a fundamental similarity 

 between the structure and the arrangement of these parts in the 

 cephalothorax of certain arthropods and in the head of verte- 

 brates, as to justify the conclusion that the vertebrates were 

 derived from fully developed arthropod types, and that the solu- 

 tion of the various problems in the morphology of the vertebrate 

 head must be sought for in the evolution of the arthropod ceph- 

 alothorax. 



For the advocate of the annelid theory of the origin of verte- 

 brates, or of any other theory that assumes the vertebrates to be 

 derived from soft bodied ancestors, an appeal to paleontology in 

 support of anatomical or embryological evidence is well nigh 

 hopeless. But for those who support the arthropod theory, such 

 an appeal is imperative because paleontology is not likely to 

 remain forever silent when both extremes of the series of hypo- 

 thetical annectant types could be preserved as fossils. 



The fossil forms that at first sight seemed most completely to 

 bridge the gap between their respective types are the trilobites 

 and Merostommata, representing the arthropods, and the ostra- 

 coderms representing the vertebrates. It seemed probable that 

 a study of their remains, especially those of the ostracoderms, 

 whose structure presents so many interesting problems, might 

 furnish evidence for, or against, the supposed genetic relationship 

 between these two groups. Moreover the fact that these ani- 



