PHYSIOLOGY OF THE INVERTEBRATA. 317 



pletely destroying physiological continuity in the rows of 

 pedicels and muscular system of the rays, does not destroy, or 

 perceptibly impair, physiological continuity in the external 

 nerve-plexus ; for however much the nerve-ring and nerve- 

 trunks may be injured, stimulation of the dorsal surface of 

 the animal throws all the pedicels and muscular system of 

 the rays into active movement. This fact proves that the 

 pedicels and muscles are all held in nervous connexion with 

 one another by the external plexus, without reference to the 

 integrity of the main trunks." 



The function of the spines and pedicellaria3 in Echinus are 

 dependent upon the external nerve-plexus ; for if the latter is 

 injured they have not the power of localising and closing, 

 round a seat of stimulation. But " other nervous connexions, 

 upon which another function of the spines depends, are not 

 in the smallest degree impaired by such injury. This other 

 function is that which brings about the general co-ordinated 

 action of all the spines for the purposes of locomotion. That 

 this function is not impaired by injury of the external plexus 

 is proved by severely stimulating an area within a closed 

 line of injury on the surface of the shell ; all the spines over 

 the whole surface of the animal then manifest their bristlinar 

 movements, and by their co-ordinated action move the 

 animal in a straight line of escape from the source of irri- 

 tation." 



It will be apparent from the above remarks that there is a 

 local reflex function of the spines and pedicellaria), which is 

 entirely dependent upon the external nerve-plexus. There 

 is also the universal reflex function of the spines, which con- 

 sists in their general co-ordinated action for the purposes of 

 locomotion, and which is entirely independent of the external 

 nerve-plexus. 



The nerves which give rise to the universal reflex function 

 are distributed over the internal surface of the shell — that is 

 they form an internal nerve-plexus. 



The internal nerve-plexus of Echinus has been recently 



