THE INSECT HEAD SNODGEASS 



465 



and rearward on a vertical or approximately vertical line of flection 

 between its base and the side or ventrolateral aspect of the body seg- 

 ment to which it was attached. It must have had, therefore, fvomotor 

 and reTYiotor muscles; and, if so, it is reasonable to assume that these 

 muscles took their origins on the dorsum and on the venter of the 

 segment supporting the appendage. We have thus a very simple pic- 

 ture of the mechanism of a primitive limb (fig. 14, Appd)^ or loco- 

 motor appendage capable of turning forward and rearward on a 

 dorsoventral axis («-&) with the body by dorsal promotor and re- 

 TThOtor muscles (/, e/), and ventral promotor and remotor muscles 

 {K^ L). A concrete example of this type of limb musculature may 

 be found in the annelid worms provided with parapodia, and like- 

 wise in the worm-like peripatids (Onychophora). From this begin- 



Stn 



Figure 14. — Diagram of tlie musculature of a primitive segmental 

 appendage 



0-6, axis of Iiasal movement of appendage on body ; Appd, append- 

 age ; I, dorsal promotor muscle ; J, dorsal remotor ; K, ventral 

 promotor ; L, ventral remotor ; Stn, sternum ; T, tergum. 



ning we may follow in our imagination the evolutionary course of the 

 appendage into a more efficient organ of locomotion with a more 

 diversified structure and mechanism. 



An appendage movable only at its base, such as the annelid para- 

 podia, can be at best only a crude organ of progression. The 

 arthropods owe their superiority over the worms to the greater 

 efficiency of their appendages. 



The first step in the development of the appendages in the primi- 

 tive Arthropoda, it would seem, must have consisted of a functional 

 division of each organ into a basis (fig. 15 A, LB), and a distal 

 shaft, or telopodife {Tlpd). The appendage as a whole having al- 

 ready a basal movement in a horizontal direction, the telopodite 

 must naturally have moved in a vertical plane on the basis, and it 

 then must have had levator and depressor muscles {0, Q) arising in 

 the basis. The baso-telopoditc joint {ct) can be identified apparently 



