84 



SMITHSONIAX MISCELLANEOUS COLLECTIONS VOL. 8l 



(Tip), and one (ft) near the middle of the telopodite. These primary 

 joints persist, evidently, as the coxo-trochanteral joint (B, ct) and the 

 femero-tibial joint (ft) of the leg {Hiiftgelenk and Kniegclenk, ac- 

 cording to Borner). The type of leg-segmentation resulting from 

 two joints so placed applies at least to the Chilopoda, Diplopoda, 

 Hexapoda, and Crustacea ; in the Xiphosura and Arachnida, however, 

 it is possible that the mechanism of the leg is given by three primary 

 joints, the second and third setting off a horizontal middle section 

 of the leg (patella). 



The further segmentation of the limb has been produced by the 

 subdivision of the principal parts of the telopodite. In the mandibu- 

 late arthropods (fig. 34 B), one or two small segments cut oft' from 

 the basal end of the j^roximal piece of the telopodite form the tro- 

 chanters (Tr), while the rest of this part becomes the femur (F) ; 

 the distal section beyond the knee joint {ft) subsegments into the 

 til)ia (Tb), tarsus (Tar), and praetarsus (Ptar). This type of seg- 

 mentation is clearly shown also in the maxillipeds or in any of the 

 anterior body appendages of Apus. In the third maxilliped (fig. 35 

 C) there are two principal flexures, one {ct) between the limb base 

 {LB) and the telopodite {Tip), the other {ft) beyond the middle of 

 the latter. The part between the two points of flexure is the femur 

 (F) with two indistinctl}' separated trochanters {Tr) ; that beyond 

 consists of two shortened segments, and the terminal praetarsus, or 

 dactylopodite. The limb base of Apus is entire, but in some arthropods 

 the basis appears to have become subdivided into a coxa (fig. 34 B, 

 Cx) and a subcoxa {Sex). The coxa may then become the free 

 fimctional base of the appendage, since the subcoxa usually forms a 

 chitinization in the pleural or the sternal wall of the segment. 



The primitive musculature of the limb base was such as to swing 

 the appendage forward and backward ; it must have comprised, there- 

 fore, promotor and remotor muscles. Probably there was a tergal 

 promotor (fig. 34 A, /) and a tergal remotor (/), and a sternal pro- 

 motor {K) and a sternal remotor (L). In a thoracic leg of an insect, 

 the base of the telopodite is provided with a depressor muscle (F) 

 having its origin on the tergum of the segment, which greatly increases 

 the lifting power of the appendage, but this muscle is not to be con- 

 sidered as a primitive element of the limb musculature. The usual 

 levator and depressor muscles of the telopodite {O, Q) have their 

 origin within the limb base. 



The simple type of musculature shown in figure 34 A, and here 

 assumed to be the primitive musculature of an arthropod limb base, 

 is actuallv present in typical form in the simpler anterior parapodia 



