THE MARINE ARACHNIDS AND THEIR ORIGIN. 341 



we know they were the first arthropods, or for that matter the very first animals 

 of any kind, endowed with the form and mechanical structure, with the sensory 

 and neuro-muscular system, adequate to perceive at a distance, to pursue, and to 

 capture living prey with measurable vigor and skill, and they finally became 

 the most rapacious and effective organisms of their time. 



But the very events that were necessary preliminaries to their active life- 

 were in the end the cause of their decline. The reduction of the oral appendages, 

 the fusion of thoracic metameres, the enlarged and condensed thoracic neuromeres, 

 gradually led, in the manner fully explained under their appropriate headings, 

 to the closing of the mouth, the inclusion of the lateral eyes in the neural tube, 

 and to other important changes that constitute the most complete metamorphosis 

 in the history of organic evolution. 



The ostracoderms, and their allies, were the products of this metamorphosis, 

 and formed the most characteristic animals of the upper Silurian. Not till 

 toward the beginning of the Devonian had they become sufficiently readjusted 

 to their new conditions to again form active and dominant organisms, or to give 

 rise to the true vertebrates. 



All the available evidence points to the conclusion that the merostomes 

 gave rise to the ostracoderms in the Ordovician or early Silurian period. 

 The constant association of merostomes and ostracoderms in the Silurian shows 

 that they lived together and were preserved under similar conditions; moreover 

 the similarity in structure and in general appearance between these two types; 

 the sudden appearance of the ostracoderms in the Silurian and their absence in the 

 preceding periods under conditions that are known to be favorable to their pre- 

 servation, as shown by the well preserved remains of their thinner skinned asso- 

 ciates of the Silurian; the culmination of the merostomes at about this period, 

 and their subsequent decline and extinction; and finally the absence of any 

 positive evidence to the contrary, admits of no other conclusion. 



II. THE OSTRACODERMS. 



The ostracoderms are so unlike any known vertebrates or arthropods that 

 very different opinions have been expressed, and are still held, as to their structure 

 and relations. It is a remarkable fact that in spite of their great antiquity and 

 simple structure, the liveliest discussions concerning them w r ere on the question: 

 Are they vertebrates or arthropods ? When the uncompromising verdict of Hux- 

 ley, and later of Lankester, was delivered in favor of the vertebrates, indeed defi- 

 nitely locating them in a highly specialized group of comparatively modern 

 teleosts, that verdict was generally accepted as final, and the morphological 

 significance that clearly belongs to them was nullified or ignored. 



Apparently no one seriously considered the possibility that the ostracoderms 

 might be very primitive vertebrates that would shed a greatly needed light on 

 the character of their remote ancestors, or that they might be annectant forms, 

 standing midway between the arthropods on one side, and the true vertebrates on 

 the other. 



