PALEONTOLOGY. 7 



appearance, but in the minute structure of their exoskeleton, the character of 

 their appendages, the arrangement of their median ocelli, and in the structure 

 of their jaws. (Figs. 232 to 265.) For a long time the ostracoderms were 

 supposed to be jawless fishes, but a special investigation of this point was made 

 and it was demonstrated that Bothriolepis, the best known member of the class, 

 possesses well developed maxillae and mandibles, quite unlike those of typical 

 vertebrates, but precisely like those demanded by the arachnid theory. 



Thus, in the light of the arachnid theory, these ancient and remarkable 

 animals, that have been repeatedly mistaken for arthropods and for vertebrates, 

 but which are neither wholly; which have withstood the keen scrutiny of Agassiz, 

 Huxley, Ray Lankester, and Smith Woodward, take on a new meaning. We can 

 now clearly see that they belong neither to the vertebrates nor to the invertebrates, 

 but form a class by themselves, intermediate between the two; presenting on the 

 one hand, in their appendages, jaws, eyes, skeleton and gills, affinities with the 

 marine arachnids, and on the other, in their tail, dermal skeleton, and dorsal fins, 

 affinities with true vertebrates. 



III. The Process of Cephalization in the Arthropods. 



If we trace the evolution of cephalization in the arthropods and analyze the 

 causes that have brought it about, we shall see that it reaches its highest expression 

 in the arachnids and that it was brought about by the same kind of changes that 

 have taken place in the vertebrates. 



A. The Grouping and the Increase in Number of Metameres. — The domi- 

 nant process in the evolution of the arthropods is the spasmodic generation of new 

 groups of terminal metameres, the gradual specialization of each group, and its 

 more intimate union with the older, more anterior members of the series. The in- 

 crease in the total number of metameres, from the first three that are characteristic 

 of the nauplius, to the seven found in the ostracods, eleven in the cladocera, and 

 the twenty-one or -two so commonly present in the higher forms, goes hand and 

 hand with the specialization and union of the more anterior groups into an increas- 

 ingly complex organic unit that in the vertebrate sense may be properly called a 

 "head." 



While this process, in a variable degree, occurs in all arthropods, it is only 

 in the arachnids that it takes place in the particular manner that is characteristic 

 of vertebrates. In the more typical representatives of that class, the first fifteen 

 or sixteen metameres are divided into unlike groups that have a similar sequence, 

 consist of a similar number of metameres, and present a similar morphological 

 and physiological specialization of organs to that seen in the corresponding regions 

 of the vertebrate head. 



It is evident, therefore, that the ancestral history of the vertebrate head is con- 

 tained in the first fifteen or sixteen arachnid metameres, and that in the arachnids 



