3i8 PHYSIOLOGY OF THE INVERTEBRATA. 



discovered by Dr. J .0. Ewart, of Edinburgh University. He 

 has found that this "internal plexus spreads all over the 

 inside of the shell, and is everywhere in communication with 

 the external plexus by means of fibres, which pass between 

 the sides of the hexagonal plates of which the shell of the 

 animal is composed." 



The nerve-centres in Echinus are to be found in the nerve- 

 ring, for as soon as the latter was removed, the animal lost, 

 completely and permanently, all power of co-ordination among 

 its spines — i.e., the function of locomotion was entirely lost. 

 Although locomotion was destroyed, the spines were not 

 entirely paralysed or motionless, for they still retained the 

 power of closing round a seat of irritation on the external 

 surface of the shell. This is due to the fact that all the 

 spines and pedicellariee are connected with the external 

 plexus, and when it is irritated, all the spines and pedicellarise 

 in the vicinity move over to the seat of irritation. " On the 

 other hand, it is the internal plexus which serves to unite all 

 the spines to the nerve-centre which surrounds the mouth, 

 and which alone is competent to co-ordinate the action of all 

 the spines for the purposes of locomotion." 



Dr. Eomanes* has shown experimentally that the am- 

 bulacral feet of Echinus are co-ordinated by the nerve-centre, 

 quite as much as are the spines. The nervous system of 

 Echinus consists of the following parts (Table, p. 319). 



Dr. L. Fredericqt has also investigated the nervous system 

 of Echinus. He finds that the pentagonal nerve-ring and its 

 five radial nerve-trunks are contained in as many sheaths, 

 which are expansions of the lining membrane of the shell. 

 The lateral branches of these nerves are also contained in a 

 similar sheath ; the latter pass out of the ambulacral pores in 

 company with the pedicels, which they serve to enervate, a 

 delicate nerve running along the whole length of each pedicel 

 to terminate at its distal end in a tactile organ. The 



* See Jellyfish, Starfish, and Sea-Urchins, pp. 307-317. 



t Archiv. de Zool. Experi. et Gin4rale, tome J, pp. 429-440. 



