338 THE OSTRACODERMS AND THE MARINE ARACHNIDS. 



Such is the nature of the problem we have before us in the present chapter, 

 and such is the method we have used in seeking an answer to it. Fortunately we 

 are dealing with animals whose size, mode of life, and abundant skeletal struc- 

 tures are highly favorable to their preservation as fossils, and paleontology should, 

 and in my judgment does, give us the materials with which this part of our prob- 

 lem may be solved. 



We may state at once that the conclusion we have reached is as follows: 

 The giant sea scorpions, or merostomata, as shown by their living representatives, 

 Limulus and other arachnids, may be regarded in a phylogenetic sense as the 

 highest arthropods, not because they now are the most highly organized, or the 

 most specialized, or the most efficient, because they are not entitled to any of these 

 distinctions, but because of all the invertebrates of their time they had made the 

 greatest progress in the attainment of that particular plan of structure that was 

 later to be so fully elaborated in the vertebrates. They were already in existence 

 in very remote pre-cambrian, or proterozoic times, and had reached a high stage 

 of development in the lower Silurian, when the ostracoderms were making their 

 first appearance, and long before any true vertebrates were known to exist. The 

 newly arrived ostracoderms had all the characteristics of an annectant class, for 

 as all authors admit, they bore a superficial resemblance, at least, in form and 

 mode of life to their arachnid contemporaries and associates, and at the same 

 time they unquestionably resembled the true fishes that were soon to appear on 

 the geological horizon. Paleontology, therefore, points very clearly toward the 

 marine arachnids as the historic predecessors of the ostracoderms, and to the 

 ostracoderms as the probable connecting link between them and the first true 

 vertebrates, such as the early dipnoi and the crossopterygians. 



I. THE MARINE ARACHNIDS AND THEIR ORIGIN. 



The most primitive arthropods were undoubtedly marine animals of the 

 short-bodied phyllopod type, consisting of a relatively large forehead, or pro- 

 cephalon, and a small body composed of a small number of ill defined metameres. 



We have seen that the foundation of segmented animals is the primitive head, 

 and that the primitive head represents the body of their remote ccelenterate an- 

 cestors. The ccelenterate, or radiate, stage is probably indicated, more or less 

 clearly, in the embryonic or larval stages of all ccelenterate derivatives. In the 

 annelids and molluscs, it is seen in the trochosphere larvae. In the arthropods 

 and vertebrates, the corresponding stages are heavily provisioned with yolk, and 

 a free larval form for this stage does not exist; but remnants of it may be recog- 

 nized in the procephalic lobes, with the included gastrula and stomodaeum. 



From the trochozoa, the hypothetical, phylogenetic antecedents of these 

 larval stages, evolution proceeds along two, possibly three, main lines. In the 

 molluscan phylum, the main theme consists in variations and specializations 

 arising in the primitive head, and in the incipient, but still unsegmented 

 trunk. In the annelids and arthropods the most important variations and 

 the most significant new characters appear in the growing trunk, while 



