642 THE AMERICAN NATURALIST.  [VOL. XXXII. 
the traces of a trilobite, and this tends to indicate that radial 
symmetry is an acquired, 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 structurally are nearer the annelids than 
Crustacea, was destined to far outnumber in individuals, spe- 
cies, orders, and classes, any other phylum. F undamentally 
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, suc- 
ceeded most admirably in this type, with the exception of the 
Trilobita, which was the first class of the phylum to appear and 
the first to disappear. The evolution of jointed limbs was 
accomplished in the most economical and direct way. The para- 
podia were perhaps utilized, and at first retaining their form in 
swimming phyllopods, afterwards from being used as supports, 
became cylindrical and jointed. All this modification of mono- 
typic forms and evolution from them to other types was accom- 
plished not very late perhaps in the Precambrian. After the 
specialization of the antennz 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 differentiation of the postantennal append- 
ages into mandibles, maxillz, 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 con- 
ditions must have been most favorable for its rapid differentia- 
tion along quite divergent lines, for in the Cambrian strata 
occur the remains of four orders, vz., the Cirrhipedia, Ostra- 
