142 THE ANCESTRY OF VERTEBRATES 



tomes from which the hypobranchial muscles are derived 

 (7th_i4th post-otic, according to Neal), afterwards consti- 

 tuting the hypoglossus. 



Afterwards the bacicward extension of the branchial basket 

 causes a secondary dysmetamerism between the gill-slits 

 and the myotomes situated above them, the number of the 

 latter surpassing that of the former. The hypoglossus by 

 this process is caused to run in a backward directed curve 

 round behind the last gill-slit to the hypobranchial mus- 

 culature. It is just in being pushed backwards by this process 

 that it collects one by one the originally post-branchial 

 ventral roots of which it is composed. In the hypobranchial 



\ 



Fig. 30. The hypoglossus of Petromyzon, a.ccord- 

 ing to Neal. 1897. 



1 primarily epibranchial ventral roots 



2 secondarily „ » » 



(hypoglossus) 



musculature, however, the correspondence of the secondarily 

 established metamerism with the arrangement of the gill- 

 slits is preserved also after the extension of the branchial 

 basket. Thus the number of muscular segments above the 

 gill slits is greater than that beneath the gill-slits (Hat- 

 SCHEK, 1892, p. 148, cf. also fig. 20). The ventral roots in 

 front of the hypoglossus-roots innervate the epibranchial 

 musculature formed from the primarily epibranchial myotomes. 

 The opposition of the four anterior segmental nerves, 

 of which the trigeminus, facialis-acusticus, glossopharyngeus 

 and vagus represent the dorsal roots, and the remaining 

 spinal nerves, has accentuated itself. Only in the former do 

 we find in most Craniates a contribution to their ganglia 

 from the lateral line system, only in the latter do we find 



