580 E. L'AV LANKESTER. 



the process of locomotion (when it takes tlie form of swim- 

 ming) is very definitely assisted by a powerful serpentine 

 movement of the whole body left and right, whilst the para- 

 podia exhibit a very rapid (far more rapid than in terrestrial 

 walking Arthropods) swinging action, the phases of which 

 are identical in the paired appendages of either side of a 

 segment, and not antagonistic in spite of the lateral undula- 

 tion of the body. 



One of the important features in the swinging movement 

 of the parapodia of Arthropoda and Chsetopoda, which can 

 be observed by simple inspection of the living animal in 

 movement^ is the fact that the number of pairs of parapodia 

 involved in a " swing-group " or (as we may put it) the 

 number which one must pass in tracing the phases oP move- 

 ment before one comes to a pair of parapodia in exactly the 

 same phase as that of the pair from which one started, varies 

 in different genera and species. Sometimes the groups may 

 be represented by a, h, c, d, e, f, g, h, a^, W, c\ d^j e^,f^, 9^, h^, 

 a2, h"-, c'-, d\ e^, f, g^, h^, where the letters of the alphabet 

 indicate a parapodium in a given phase of swing, and a in the 

 first group is identical in phase with a} in the second, with a^ 

 in the third, and so on. In other cases the groups are repre- 

 sented by two units only — a, h, a}, h^, a^, b-, and so on. 



In P. capensis (PI. 42, fig. 4) the swing-group number is 

 only two, a. h. The anterior unit a swings forward, whilst 

 the posterior unit h has its claws grasping the surface, and is 

 swinging backwards. As soon as parapodium a approaches 

 parapodium W- (and similarly throughout the series) the 

 movement changes, a grasps the surface, and h^ (and all the 

 others corresponding to it, viz. h, h^, ly", h^, h^, ?/') lets go and 

 commences to swing forward. This is shown in the figures 

 4, 5, and 6 of PI. 42. 



In the Millipede Archispirostreptus, on the other hand, 

 the swing-group number is sixteen, and (as our figures 1 

 and 2 of Plate 42 show) there are eight of these groups, 

 allowing for peculiarities in the extreme anterior and posterior 

 somites. The regions indicated by the lettering a to /in the 



