STRUOTURB AND CLASSIFICATION OP THE ARTHBOPODA. 579 



photography, aud require also the consideration of a large 

 number of forms^ such as a representative series of marine 

 Ch^topoda, several genera of Diplopoda, and of Chilopoda, 

 the Phyllopod crustaceans and the higher forms, Hexapod 

 insects and larva3. 



The most important fact which the drawings here published 

 show is that in Peripatus and the Millipede the limbs on 

 opposite sides of the body, which are morphologically related 

 as " pairs," are always in the same phase of fore-and-aft 

 swing; they move together and identically. On the other 

 hand in the Centipede the pairs or opposite limbs on a 

 segment are in phases, which ai'e the extreme opposites in 

 the series of positions through which the limb swings. 



Further, it is to be noted in connection with this that the 

 strongly chitinised body of the Millipede takes no part by 

 serpentine movement in the locomotory process; it remains 

 perfectly straight. So, too, the soft body of Peripatus — 

 though it is frequently bent and turned on itself, and may be 

 more or less elongated and contracted at various intervals, 

 yet does not contribute by any serpentine ^' stroke " to the 

 process of locomotion. Ou the other hand the Centipede's 

 locomotion is very largely effected by a powerful lateral 

 undulation of the body — groups of three segments being 

 alternately slightly tilted by muscular contraction first on one 

 side and then on the other. 



In the case of the Centipede, as already noted, this 

 serpentine rhythmic movement of the body is accompanied 

 by an opposition in the phase of the swing- movements of those 

 legs which are paired with one another in a single segment, 

 and a special kind of leg and body movement is the result, 

 with which the special forms of leg-rhythm producing loco- 

 motion in other highly-developed Arthropoda (including the 

 tripod action in Hexapoda) might be compared with a view to 

 a mechanical explanation of their genesis. 



On the other hand it is worth calling to mind that in some 

 of the large marine Chtetopoda, viz. in Nephthys and Nereis 

 (very few observations on the subject have been recorded) 



