78 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



Stock, which arose from some generahzed nonchaetopodous anneHd; 

 but since none of the modern anneHds has these characters it is 

 evident that the annehdan progenitors of the Protonychophora- 

 arthropoda have left no direct descendents. The Arthropoda differ 

 in so many respects from present-day Onychophora that it is certain 

 they must have branched off from the common onychophoran- 

 arthropod trunk before the latter had gone far in the onychophoran 

 direction. Arthropod forms were highly developed and differentiated 

 in the early Cambrian period of geological history, and must, therefore, 

 have had their origin in remote pre-Cambrian times, though in the 

 rocks of this period there is no specific evidence of their existence. 

 As an individualized group, the Arthropoda are characterized 

 particularly by the development of hard plates in the cuticular layer 

 of the integument, separated by areas of flexibility. In the Mandib- 

 ulata sclerotization results from the presence of nonchitinous sub- 

 stances in the otherwise chitinous cuticula; in the Trilobita and 

 Chelicerata sclerotization may be due to a structural differentiation 

 of the chitin itself, though apparently little attention has been given 

 to the chemical composition of the cuticular skeleton in these groups. 

 Ruser (1933) describes the physical structure of "hard chitin" and 

 "elastic chitin" in the Ixodidae, but makes no determination of their 

 chemical nature. 



Since the muscles are primarily attached on the body wall, the 

 differentiation of the latter into hard and flexible areas at once created 

 a possibility for unlimited development of skeletomuscular mecha- 

 nisms, and it is through the elaboration of such mechanisms that the 

 arthropods have attained their exalted position among the articulates, 

 and their wonderful diversity of structure. It is true, of course, that 

 some of them, particularly those that have taken up parasitic habits, 

 have renounced their birthright, and among the latter we find examples 

 of physical degeneration carried to such an extent that every semblance 

 of arthropod structure may be lost. 



Sclerotization of the integument involved first a complete change 

 in the mechanism of body movement, for if the rings of flexibility 

 between segmental plates remained at the primary intersegmental 

 grooves, on which the longitudinal muscles are attached, there would 

 be little if any possibility of movement. Hence, each dorsal and 

 ventral plate includes the primary intersegmental groove in front, 

 while the areas of flexibility occupy the posterior parts of the seg- 

 mental regions. The sclerotized parts of the primary intersegmental 

 grooves, carrying the muscle attachments, thus come to form internal 

 ridges, or antecostae, on or near the anterior margins of the definitive 



