PHYSIOLOGY OF THE INVERTEBRATA. 



315 



axis and the bases of the mandibles (Fig. 56, 1', and Fig. 60). 

 '^riie plexus, now in the form of exceedingly delicate fibres 

 connecting small bipolar cells, reaches the special muscles 

 of the mandibles Although this plexus is especially 





^*^«^^:HS^^XZ^.'=t^?^^i^;i^--^-^^==^^^^^^as^^ 



Fig. 60. — Xervf.-plexus lying ovkr Muscular Fibres near Base 

 OF Mandibles ov Pediceli.aria. 



(^After Romanes and E\vart.) 



related to the muscular fibres — lying over and dipping in 

 between them — it is also related to the surface epithelium, 

 and delicate fibres often extend from it to end under or 

 between the epithelial cells." 



Romanes and Ewart have shown that the Ecliinodermata 

 respond to all kinds of stimulation. The pei'iod of latency 

 varies considerably in different species, and in different parts 

 of the same animal. 



"The external nerve-plexus supplies innervation to three 

 sets of organs — the pedicels, the spines, and the pedicellaria? ; 

 for when any part of the external surface of Echmns is 

 touched, all the pedicels, spines, and pedicellarice within 

 reach of the point that is touched immediately approximate 

 and close in upon the point, so holding fast to whatever body 

 may be used as the instrument of stimulation. In executing 

 this combined movement the pedicellarite are the most 

 active, the spines somewhat slower, and the pedicels very 

 much slower. If the shape of the stimulating body admits 

 of it, the forceps of the pedicellarias seize the body and hold 

 it till the spines and pedicels come up to assist." 



The function of the pedicellariae is to aid locomotion by 



