466 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



in the limbs of all present-day arthropods by the uniformity of its 

 movement and musculature. 



The next step in line with greater mechanical improvement in the 

 appendage produced a point of flexure near the middle of the telo- 

 podite (fig. 15 B, ft)^ enabling the distal part of the latter to be 

 more effectively brought down against the support. Hence, in all 

 fully developed arthropod limbs there is a " knee " joint (C, ft) in 

 the telopodite with a principal downward movement of the part 

 beyond the knee. 



FiGDRE 15. — Diagrams of segmentation of arthropod legs 

 A, primary division Into basis (LB) and telopodite (Tlpd) at coxo-trochan- 

 teral Joint (ct). B, division of telopodite at knee joint (ft). C, com- 

 plete segmentation of an insect's leg. D, a typical arachnid leg. a— 5, 

 axis of limb basis on body; ct, coxo-trochanteral joint; Cx, coxa: Ftn, 

 femur ; ft, femoro-tibial joint ; LB, limb basis ; O, levator muscle of 

 telopodite ; Pat, patella ; pt, patello-tibial joint ; Ptar, praetarsus ; Q, 

 depressor muscle of telopodite ; Sex, subcoxa ; Tar, tarsus ; Th, tibia ; 

 Tlpd, telopodite ; ITr, first trochanter ; 2Tr, second trochanter. 



We may thus conceive of the early arthropods as being centipede- 

 like creatures with a series of legs on each side of the body, the legs 

 all jointed in the same way, and moving by a uniform kind of motion. 

 The appendages all turned forward and rearward on the body ; they 

 all turned upward along the line of the baso-telopodite joints; and 

 the distal parts uniformly bent downward at the knee joints. Subse- 

 quently the major parts of the telopodite of each limb have been stili 

 further segmented (fig. 15 C, D), and in some arthropods the basis, 

 too, appears to have been subdivided (C, LB). 



