404 COMPARATIVE ANATOMY AND. PHYSIOLOGY. 



fetill retain a very close connection with the surface 

 bi the' body ; the Annulata, for example, present us 

 with various arrangements of this kind, for while 

 Chsetopterus and Spio have the nerve cords out- 

 side the muscular layer of the body wall, and others, 

 such as Hermella, have them between or even in 

 the substance of these muscles, others again, like 

 the earthworm, have them placed inside the muscular 

 layers. 



In the simplest condition of those form's which do 1 

 not present the most primitive arrangements^ we find 

 a central gaiiglionic, or cerebral mass* with 

 which there are connected a number of nerve fibres, 

 which pass to different parts of the body ; such i 

 disposition is found in some of the Turbellaria, and in 

 the .Rotatoria. 



The most important advance is seen in the appear- 

 ance of the main or longitudinal cords, such as we have 

 already noted in Peripatus ; but even when these do 

 appear, we find that the cerebral mass still gives off a 

 number of fibres, which pass to the different sensory 

 organs that are situated at the anterior end of the 

 body. The two main trunks that pass backwards are 

 more or less intimately connected with one another on 

 the ventral surface of the gullet, so that we have now 

 to distinguish the carebral, or supracBsophageal 

 ganglia, the cesopliageal nerve cords or com- 

 missures, and the sufooesopliageal ganglia; 

 these last are, in their most primitive conditions, 

 similar to those that follow them (Fig. 174) ; at 

 first they are not closely united with one another, 

 but connected together by a pair of transverse com- 

 missural cords, as are the ganglia that follow them. 



In the more primitive conditions, such as are 

 presented by Apus among the Crustacea, the cerebral 

 ganglia are merely formed by the nervous swel- 

 lings in the anterior region (primitive cerelrum) 



