340 THE OSTRACODERMS AND THE MARINE ARACHNIDS. 



to the shell gland, or mantle, or to embryonic membranes, or branchiocephalic 

 folds. 



The earlier stages in the evolution of arthropods can only be inferred, as 

 indicated above, from the records of comparative anatomy and embryology. 

 The original phylogenetic records are lost beyond recovery, for in the oldest 

 rocks in which any formal organic remains aie found, the eurypterid type appears 

 to be already present. These fragmentary remains, Beltina danai, consisting 

 of tracks, outlines of heads and appendages, were found by Walcott 1 nine 

 thousand feet below the uncomformity between the Proterozoic (Algonkian) and 

 the Cambrian. 



During the long subsequent period, including the Cambrian and Ordovician, 

 the familiar types of marine arthropods, such as the short-bodied bivalve ostra- 

 codes and the shield-covered phyllopods and phyllocarids, make their appearance 

 in increasing numbers. Trilobites and merostomes are likewise found in great 

 abundance and variety, the former reaching their climax in the Ordovician, the 

 latter in the Silurian and early Devonian. 



During this long period, organic evolution proceeded very slowly and there 

 are no indications that any single event took place of exceptional importance 

 morphologically. Nevertheless important progress was made in the arthropods 

 in those complex processes of local suppression, union, and enlargement of mul- 

 tiple organs, that are such essential features of all progressive organizations, and 

 which alone could make a more active, varied, and efficient mode of life in seg- 

 mental animals a possibility. This kind of reorganization tends to convert the 

 linear succession of like metameres, each complete in itself and independent of 

 the others, into a linear succession of unlike organs, each subordinate to all the 

 others and all together forming a new organism of a much higher order. See 

 Chapters I and II. 



While the most important event of the Proterozoic period was no doubt the 

 outgrowth from the radially symmetrical ccelenterate of a new bilaterally symmet- 

 rical trunk, and the perfecting in it of a high degree of metamerism, in the Cam- 

 brian and Ordovician the important events were the breaking down of this meta- 

 merism, especially in the older and more anterior metameres, and the successive 

 merging of its various parts into a more efficient aggregate, the cephalo-thorax, 

 that at a much later period was to become an important part of the complex 

 vertebrate "head." 



The trilobites, judging from strategraphical evidence and from their ex- 

 ternal form, remained what they had been almost from the outset, slow-moving, 

 sea-bottom feeders, like Limulus to-day, making only occasional, apparently aim- 

 less excursions, as free swimmers, into the water above. 



But most of the merostomes in the manner above indicated had acquired, 

 at a very early period, a superior organization in the head region that enabled 

 them to leave the bottom and swim at large with purpose and effect. So far as 



1 Bull. Geol. Soc. Am., Vol. X. 



