SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



body wall could not occupy the entire length of each segment, for if 

 they did the creature would become a tube that might be compressed 

 or expanded dorsoventrally but that could not move otherwise. A 

 circumferential part of each segment must, therefore, remain flexible. 



A 



B 



C 



Fig. 2. — Diagrammatic lengthwise sections of a segmented animal, showing 

 primary and secondary segmentation. 



A, primary segmentation of soft-bodied animal, with segments (Seg) marked 

 by intersegmental rings (Isg) to which longitudinal muscles (LMcl) are 

 attached. 



B, secondary segmentation of insect with hard plates in its body wall — each 

 tergum or sternum (T, S) includes part of chitinized intersegmental groove 

 before it, forming antecosta (Ac) internally and antecostal suture (ac) ex- 

 ternally, with a narrow precosta (Pc) on anterior margin; posterior unchitinous 

 part of segment becomes secondarily the "intersegmental" membrane (Mb). 



C, secondary segmentation accompanied by telescoping of the segments, each 

 segmental plate ending in a posterior fold, or reduplication (Rd). 



The flexibility could not well be at the intersegmental lines, because 

 the muscles are here attached and demand a firm support, for which 

 reason the dorsal and ventral parts of the intersegmental grooves 

 have been hardened and converted into internal ridges (fig. 2 B, Ac), 

 each marked externally by a corresponding groove, or suture (ac). 

 That each ridge should become continuous with the plate behind it 



