Chap. X.] -LEGS OF ARTHROPODA. 377 



the penultimate segment of the body, which, in place 

 of being comparatively small parts, as in the typical 

 third abdominal appendage, are widened out into 

 more considerable plates, which have a backward 

 instead of a downward direction ; these unite with 

 the terminal segment, which sometimes, though very 

 rarely (Scyllarus), bears minute appendages, to form the 

 powerful flapper of the Crayfish. 



Peripatus, the species of which vary considerably 

 in the number of appendages, have these organs only 

 imperfectly jointed, and they move but slowly ; in 

 them, as in all Arthropods other than the Crustacea, 

 the limbs are uni-, and not bi-ramose, but, as often 

 happens, they are provided with a terminal claw. 



The Myriopoda (Centipedes), as their name 

 indeed implies, have a large number of walking limbs, 

 each of which has essentially the same characters as 

 that which precedes and that which follows it ; in 

 the Millipedes a number of segments carry two pairs 

 of legs each. The Araclmida have four pairs of 

 walking limbs, which are completely lost in such 

 endoparasitic forms as Pentastomum. The Insects, or 

 as they are very appropriately called, the Hexapoda, 

 have three pairs of walking limbs ; these are typically 

 composed of a coxa, a trochanter, a femur, a tibia, 

 and a six-jointed tarsus, which ends in a pair of claws ; 

 the larval or caterpillar forms have, however, a more 

 or less larger number of walking appendages, or pro- 

 legs ; these are best and most numerously developed 

 among the Lepidoptera, but they are in all cases 

 rudimentary as compared with the legs of the adult. 



A large number of insects have yet another set of 

 locomotor organs, in the shape of the dorsally-placed 

 wings ; of these there are never more than two pairs, 

 and of these both may be rudimentary, as in the 

 female cockroach ; or the anterior pair only may be 

 developed as in the Diptera (flies), or the hinder alone 



