i8o 



COMPARATIVE ANATOMY 



formed of a dense mass of muscle divided into a number of 

 squarish blocks by connective tissue septa. In a section taken 

 behind the atriopore these muscular masses extend down the 

 sides of the body to the origin of the ventral fin, but in a 

 section through the pharyngeal region they only form the 

 upper part of the side walls of the atrial chamber, the lower 

 walls and floor being formed by the thin muscular sheets of 

 the atrial folds. It is obvious, on reflection, that these muscle 



Fig. 43 



A. Transverse section taken a little way in front of the atriopore of Amphioxus. 

 B. Transverse section taken between the atriopore and the anus, ao, dorsal 

 aorta ; at, atrium ; coe, coelom ; df, dorsal fin ; en, intestine ; ep, epipleur \fr, 

 fin-ray ; my, myotomes ; mp, metapleural fold ; nek, notochord ; nt, neural tube ; 

 si, subintestinal veins ; vf, ventral fin. 



masses are the myotomes of which several, because of their 

 V shape, are cut across in one section, and for the same reason 

 each myotome is cut twice. 



In a section taken just in front of the atriopore, posterior to 

 the hepatic caecum (fig. 43, A\ the intestine is seen suspended 

 by a fold of tissue from the lower part of the notochordal 



