STEUCTtJKE AND CLASSIFICATION OF THE AKTHliOFODA. 581 



figure are regions where parapodia exhibit the extreme 

 forward swing-phase. They may be called "group-crests/' 

 Group-crests are but " phases " in the swinging of the limbs^ 

 and they pass along the whole series from behind forward, 

 like the crest of a wave passing along a liquid, l^ach pair of 

 successive parapodia is in turn the seat of the group-crest, 

 and the waves keep Howing from behind forward with 

 beautiful regularity. 



The rate should be measured in different forms, and the 

 conditions affecting the rate of this rhythmic movement should 

 be studied experimentally. 



In the Centipede (PI. 42, fig. 3) the " swing-group " 

 number appears to be six, and the whole phenomenon is 

 profoundly modified by the fact that lateral undulations of 

 the body itself are a definite part of the locomotor activity, 

 whilst the limbs on opposite sides of the same segment are 

 not identical, but antagonistic in phase. 



It seems to me probable that the condition presented by 

 the Centipede is a much higher development than that seen 

 in the Millipede, and implies a unilateral differentiation of 

 muscles and nerves which is far from primitive. It may, I 

 think, be reckoned as one of the characters tending to 

 separate the Diplopoda or Prosthogonopora altogether from 

 association with the Chilopods. It would, of course, be very 

 interesting in this connection to have some reliable photo- 

 graphic studies of the phases of parapodial swing in such 

 forms as iScutigera, and, indeed, in all families of Chilopoda. 



EXPLANATION OF PLATE 42, 



Illustrating Professor Lankester's article on the Arthropoda. 



Pig, 1. — Lateral view of a specimen of Arcliispirostrept us pyro- 

 cephalus (de Koch) drawn from a living specimen in moveineut. Magnified 

 twice linear. 



