96 ANATOMY OF AMPHIOXUS. 



very sharply defined parts (Fig. 39) ; namely, (i.) the cerebral 

 or supracesophageal ganglion, which is situated dorsally 

 over the buccal cavity; (ii.) a longitudinal solid nerve-cord 

 composed of two more or less distinct halves, running 

 along the whole length of the ventral side of the body 

 below the alimentary canal ; (iii.) the circumcesopJiageal 

 nerve-ring or commissure which encircles the buccal tube 

 and connects the cerebral ganglion with the snbo2sophageal 

 ganglion at the anterior extremity of the ventral nerve- 

 cord. 



Viewed from above (as in Fig. 39), the ventral nerve- 

 cord presents a series of constrictions which are in some 

 forms very pronounced. The wider portions occur in the 

 middle of the body-segments, and constitute the ventral 

 ganglia, which are strung together by the intervening 

 nerves (connectives) in the form of a ganglionic chain. 

 From the ganglia, paired nerves pass out to the organs of 

 the body. 



One of the greatest peculiarities in the type of nervous 

 system above described lies in the fact that the alimentary 

 canal passes through and is surrounded by a portion of 

 the central nervous system ; namely, the circumoesophageal 

 commissure. This fact has been one of the most serious 

 difficulties which the upholders of the Annelid-theory have 

 had to contend with. 



In the Chordata the alimentary canal does not pierce 

 the central nervous system in any sense whatever.* Never- 

 theless, there have been many conjectures as to a possible 

 equivalent of the circumoesophageal nerve-collar in the 

 Vertebrates, although it is safe to say that nothing of the 

 kind really exists. 



* Balanoglossus might be said to offer an exception to this rule (see 

 Chap. V.). 



