NO. 3 



INSECT HEAD SNODGRASS 



83 



the Crustacea represent a group of arthropods that have secondarily 

 adopted an aquatic life, and that, while certain forms have become 

 thoroughly adapted to a free life in the water, others have retained, 

 with but little modification, some of the organs that were developed 

 primarily for terrestrial locomotion. This view, perhaps, is contrary 

 to generally accepted ideas concerning the evolution of the arthropods, 

 but it is clearly futile to attempt to derive the appendages of arth- 

 ropods in general from the swimming appendages of Crustacea. 



If the ambulatory limb be taken as more nearly representative of 

 the basic structure of an arthropod appendage than is the natatory 



Fig. 34.- 

 malic. 



-Generalized segmentation and musculature of an insect leg, diagram- 



A, theoretical segmentation and musculature of a primitive arthropod leg, 

 anterior view: the appendage, consisting of a limb base (LB), and a telopodite 

 {Tip) of two segments, moved forward and backward on vertical basal axis 

 {a-b) bv tergal and sternal promoters (/. K) , and tergal and sternal remotors 

 U,L).' 



D, definitive segmentation of an insect leg by division of limb base (A, LB) 

 into subcoxa {Sex) and coxa {Cx), and by subsegmentation of first part oi 

 telopodite into trochanter and femur, and of second part into tibia, tarsus, and 

 praetarsus. 



a-b, basal axis of limb base ; ct, coxo-trochanteral joint ; Cx, coxa ; F, femur ; 

 ft, femoro-tibial joint; /, tergal promotor; /, tergal remotor; K, sternal pro- 

 motor ; L, sternal remotor ; O, levator of telopodite ; P, tergal depressor of telo- 

 podite (characteristic of insects) ; Ptar, pretarsus; Q, depressor of telopodite; 

 Sex, subcoxa ; T, depressor of distal segment of telopodite ; Tar, tarsus ; Tb. tibia. 



limb, we have only to inquire as to what was its probable form in the 

 ancestors of terrestrial arthropods. The primitive appendage un- 

 doubtedly turned forward and backward on a vertical axis through its 

 base (fig. 34 A, fl, &), as does the parapodium of a modern polychaete 

 annelid (fig. 33 A, B, C). For walking purposes, however, the limb 

 must have acquired joints, and, as Bonier (1921) has shown, the 

 simplest practical condition would demand at least tzvo joints with 

 vertical movements (fig. 34 A), one near the union of the leg with the 

 body {ct), dividing the limb into a basal piece {LB) and a telopodite 



