6 OUTLINE OF THE ARACHNID THEORY. 



racic and cephalic region, and the increasingly precocious development of the fore- 

 brain, would inevitably lead to the formation of a more pronounced head fold, 

 with a disproportionately shortened or diminished haemal surface, and would force 

 the bases of the more anterior oral appendages forward and haemally till they 

 meet on the opposite side of the head, thus giving rise to thepremaxillary, maxillary, 

 and mandibular arches of the vertebrate head. d. This shortening of the ante- 

 rior haemal surface of the head inevitably draws the heart, with its neighboring 

 muscles and nerves derived from the vagal and branchial segments, farther for- 

 ward into the head region, thus producing that remarkable forward dis- 

 location of the heart, hypobranchial muscles and nerves so familiar in vertebrates. 

 (Figs. 17, 19, 33, 77.) e. Finally the readjustment of the whole head, in response 

 to these changes, leads to that new condition of architectural stability that marks 

 the true vertebrates. 



The preliminary stages that lead up to these readjustments were, no doubt, 

 gradual and more or less tentative, for they did not in themselves create sufficiently 

 altered conditions to upset the balance of organic power. But the later stages 

 of the readjustment, especially the final stages in the transfer of the oral arches to 

 the haemal side, appear to have been rapidly accelerated for a period and then 

 checked by their approaching reunion on the haemal side of the head and by the 

 creation there of a new condition of organic stability. 



The closing of the old mouth, the formation of a new one, the transfer of the 

 oral arches to the haemal side, and the appearance of true gill clefts must have 

 taken place during the embryonic, or larval period, the increasing volume of the 

 yolk sphere making such a cataclysmic metamorphosis possible. Hence it is 

 probable that the transition from the arthropod to the vertebrate type will never 

 be completely bridged by the discovery of new animals. The gap between the 

 two classes is a real one, representing a comparatively short period of rapid trans- 

 formation from the old condition in the arthropods to a new, approximately 

 stable condition in the vertebrates. 



D. Paleontology. Nevertheless, we shall show that the wide gulf which now 

 separates the arachnids and vertebrates, in some important respects, was bridged 

 in early paleozoic times by a large and varied class of animals known as the 

 ostracoderms. They constitute the only great class of animals that have flour- 

 ished for a comparatively short period and then become totally extinct; a 

 fact that in itself testifies to the unstable, transitory character of their anatomical 

 structure. 



Heretofore it has been assumed that the ostracoderms were highly specialized 

 vertebrates, in spite of the fact that they possessed a very simple and primitive 

 structure, and were the first vertebrate-like animals to appear on the geological 

 horizon. They were contemporaneous with the highest and most dominant 

 type of arthropods then in existence, the marine arachnids, or sea scorpions, of 

 the Silurian period. There is a striking resemblance between these early verte- 

 brates and the contemporaneous arachnids, not only in their form and general 



