254 



SCIENCE. 



[N. S. Vol. YIII. No. 191. 



skin, the parapodia, which served as swim- 

 ming organs. Other nepionic forms, at first 

 free swimming, became fixed and protected 

 by two valves as in the Brachiopoda, whicli 

 owe their success in Precambrian times to 

 to tlieir fixed and protected bodies. 



Not long after the annelid type became 

 established that of the echinoderms ap- 

 parently diverged from some nepionic worm, 

 like a ti'ochosphere. In such a form there 

 was a tendency to the deposition of particles 

 and plates of lime in the walls of the body, 

 and the type, becoming fixed at the bottom, 

 or at least nearly stationarj^, and mean- 

 while more or less protected by a calcareous 

 armor, lost its originally bilatei-al and ac- 

 quired a radial symmetry. 



But no echinoderms have yet been de- 

 tected in Percambrian rocks, which, how- 

 ever, have revealed arthroj)ods, as shown 

 by the traces of trilobites, and this tends to 

 indicate that radial symmetry is an ac- 

 quired, not a primitive characteristic. 



At this time was solved the problem of 

 the origination of a type of body, and of 

 supports for it either in walking or in 

 swimming, which should fulfill the most 

 varied conditions of life, and this type, the 

 arthropodan, as events proved, was that 

 fitted for walking over the sea-bottom, for 

 swimming or for terrestrial locomotion ; 

 nor was the idea of segmentation both in 

 trunk and limbs discarded when the type 

 culminated in flying forms — the insects. 



The Arthropoda, as the record shows, 

 first represented by trilobites, which struc- 

 turally are nearer the annelids than Crus- 

 tacea, was destined to far outnumber in in- 

 dividuals, species, orders and classes any 

 other phylum. Fundamentally worm-like 

 or annelid in structure, the body consisted 

 of a linear series of stiff levers, and was 

 supported by limbs segmented in the same 

 way. The variations of the arthropodan 

 theme are greater than in any other groups, 

 and nature, so to speak, succeeded most ad- 



mirably in this type, with the exception of 

 the Trilobita, which was the first class of 

 the phylum to appear and the first to disap- 

 pear. The evolution of jointed limbs was 

 accomplished in the most economical and 

 direct way. The parapodia were perhaps 

 utilized, and at first retaining their form in 

 swimming phyllopods, afterwards from be- 

 ing used as supports, became cylindrical 

 and jointed. All this modification of mono- 

 typic forms and evolution from them to 

 other types was accomplished not very late 

 perhaps in the Precambrian. After the 

 specialization of the antennae and of the 

 trunk-segments of the trilobites was worked 

 out, all the postantennal appendages being 

 alike, there ensued in some descendant of 

 another vermian ancestor a further difieren- 

 tiation of the postantennal appendages into 

 mandibles, maxillse, maxillipedes, thoracic 

 ambulatory legs, and abdominal swimming 

 feet, as worked out in the more specialized 

 members of the class of Crustacea. 



As soon as the crustacean type became 

 established, the conditions must have been 

 most favorable for its rapid differentiation 

 along quite divergent lines, for in the Cam- 

 brian strata occur the remains of four orders, 

 viz., the Cirrhipedia, Ostracoda, Phyllo- 

 poda, the sole Cambrian form (Protocaris 

 marshi related to the modern Apus), and the 

 Phyllocarida. Of these the barnacles and 

 ostracodes, with their multivalve or bivalve 

 carapaces, are the most specialized, and in 

 the case of the former the process of modifi- 

 cation due to this fixed mode of life must 

 have required ages, as must also the devel- 

 opment of that highly modified vermian 

 type, the Brachiopoda. 



Indeed, the three lines of descent which 

 resulted in the arthropodan phylum, as it 

 now exists, unless there were three inde- 

 pendent phyla, were perhaps initiated be- 

 fore the Cambrian. These lines are (1) 

 the Trilobita, with their probable successors 

 the merostomes and arachnids, (2) the 



