84 ADRENALIN IN ANNELIDS 



contractile and has physiological actions in every way comparable to 

 those of the vertebrate heart. It is controlled by similar nerves and 

 reacts to the drugs adrenahn, atropine, muscarine, and curare in an 

 identical way. The two main vessels, which lie in the extreme lateral 

 position in the body just under the longitudinal muscle layers, are 

 also similar in function. Their chief purpose is to drive blood into an 

 extensive capillary network which lies in the skin and is respira- 

 tory in function. The condition in Eunice giganka is probably a prim- 

 itive form of that in the leech. The segmentally arranged muscu- 

 lature which surrounds a short portion of the vessel lying at the base 

 of each branchia has become more diffusely spread and has fused to 

 form one continuous contractile vessel on each side. In Eunice again 

 the function of these "hearts" is entirely branchial. The growing 

 around of the lateral folds of the invertebrate to form the ventral 

 surface of the vertebrate, which is hypothecated in the theory of the 

 origin of vertebrates brought forward by W. H. GaskelV^ would carry 

 with them the two lateral vessels. They would thus become mid- 

 ventral and would lie in the position of the two vessels from which 

 embryologically the vertebrate heart is formed. The physiological 

 function of such a heart would be always branchial from its earliest 

 origin in the annelid kingdom. 



The formation of a specialized vascular muscle immediately de- 

 manded the formation of special nervous and chemical regulators to 

 this muscle; we therefore find a segmental nervous control already 

 estabhshed in annelids for the vascular muscle of each segment, and 

 a speciaHzed cell which secretes the necessary internal secretion; 

 namely, adrenaHn. In this primitive condition the secretion of adre- 

 nalin is a function of nerve cells, constant in number, which are situ- 

 ated in the segmental ganghon and are also in all probability the 

 nervous regulators of the vascular muscle. These cells therefore rep- 

 resent the common ancestors of the sympathetic and of the adrenalin- 

 secreting systems of the mammal. In the course of evolution the 

 two functions have become separated, and two distinct types of cell 

 have arisen, one of which is purely secretory and the other purely 

 nervous. In the earliest vertebrates the secretory system is chiefly 



" Gaskell, W. H., The origin of vertebrates, London, 1908. 



