1 60 THE INVOL UNTAR Y NER VO US S YSTEM 



throughout its whole length, and afterwards became more and 

 more confined to the parts where sphincter muscles were neces- 

 sary and advantageous. 



Lastly, I will consider the origin of the system of muscles 

 derived from those surrounding the segmental duct. The ex- 

 cretory organs of vertebrates are universally considered to have 

 been derived from the segmental excretory organs of annelids, 

 and in the leech these excretory organs connect with the exterior 

 by a duct in each segment. This duct has well-defined muscular 

 walls and possesses in its course an enlargement, the so-called 

 nephridial vesicle or bladder, with contractile walls. It is natural 

 therefore to look upon the segmental duct system of muscles as 

 originating from this group of muscles, and therefore in a separ- 

 ate category from either the endodermal or ectodermal muscles. 

 This might account for the absence of any sign of reciprocal in- 

 nervation in this muscular group, and its innervation with both 

 motor and inhibitory nerves from cells of the sympathetic system 

 only. In consideration of the marked manner in which this 

 muscular system in the vertebrate responds to adrenaline, I should 

 expect to find these muscles in the leech also respond to adrenalin ; 

 that is a question worth investigation. It is possible and indeed 

 probable that the segmental excretory ducts are ectodermal 

 both in the annelid and the vertebrate, in which case this mus- 

 cular system would belong to the ectodermal system of muscles 

 in both cases. 



