76 
tomes from which the hypobranchial muscles are derived 
(7tE-14 post-otic, according to NEAL), afterwards consti- 
tuting the hypoglossus. 
Afterwards the backward 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 ís 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 
ä 
Vu 
e 6 5 Ee 
EEN Pe en rn 
Fig. 30. The hypoglossus of Petromyzon, accord- 
ing to NEAL, 1897. 
1 primarily epibranchial ventral roots 
2 secondarily 5 s 
(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. 
e 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 fin 
