PHYSIOLOGY OF THE INVERTERRATA. 389 



sand and pieces of sliells, or " by secreting a cbitinous or 

 calcified slielly substance." 



Locomotion in the Annelida is aided bv means of aciculi 

 and seta?. These are used as fulcra when they creep, or as 

 oars when they swim. 



The Nematoscolices. 



The general movements of the body in the Ncviatoidea 

 are due to a subcutaneous circular muscle-layer; and its 

 longitudinal and transverse muscles are quite distinct from 

 each other. In most of the Ncmatoidca the longitudinal 

 muscles form four large bands — tvvo on the ventral and two 

 on the dorsal surface. These animals are devoid of limbs, 

 though they may sometimes be provided with setiform spines 

 or papilltc. 



The Myriapoda. 



Great advance is observed in the mode of locomotion in 

 the Arthropoda. There is no longer any contractile envelope 

 like the Anndidn and their allies, for here the muscular fibres 

 are all grouped into distinct masses or individual muscles, 

 which are inserted into such and such a limb or part of the 

 body by means of tendons. In the Arthropoda all the muscles 

 are transversely striated. 



Locomotion in the Myriapoda is produced by means of the 

 limbs. Almost all the segments bear at least a pair of 

 articulated limbs terminated by claws. 



The Cliilopoda, (centipedes) usually live in the earth or 

 under stones ; they run with considerable swiftness in pursuit 

 of their prey, and can even progress backwards by the aid of 

 their tail-like posterior limbs, which at other times are 

 dragged helplessly behind them. 



The Dij)lopoda or Cliiloynatlia (millipedes) possess two pairs 

 of limbs on each segment except the posterior segment, which 

 is devoid of these organs. The movements of these animals, 



