PHYSIOLOGY OF THE INVERTEBRATA. 



315 



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

 The 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 



Fig. 60. — Nerve-plexus lying over Muscular Fibres near Base 

 OP Mandibles of Pedicellaria. 



(^After Romanes and Ewart.) 



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.'' 



Eomanes and Ewart have shown that the Ecliinodermata 

 respond to all kinds of stimulation. The period 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 pedicellariEe ; 

 for when any part of the external surface of Echinus is 

 touched, all the pedicels, spines, and pedicellarise 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 pedicellariEe 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 pedicellarise seize the body and hold 

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



The function of the pedicellarise is to aid locomotion by 



