17 
as a rule a separation of the (visceral) motor part from 
the ganglion in the form of the sympathetic gangiia. Thus 
the four head ganglia contain two elements which are absent 
in the spinal gangiia, probably even three, if we count 
the epibranchial placodes as a third system. The first 
segmental nerve, the trigeminus, just as in Amphioxus, has 
a double character and supplies, besides its own segment, 
also the prostomium or snout with sensory branches. 
Something similar is the case with the second pro-otic- 
nerve, the facialis-acusticus, which supplies the cephalic 
part of the lateral line (including the prostomial part) The 
main part of the lateral line extending along the trunk is 
supplied by the ramus lateralis vagi which probably may 
be considered as a collector which, according to EISIG 
(1887) and HATSCHEK (1892, p. 151-152), has collected the 
lateral part of the dorsal branches of the dorsal spinal nerves. 
While in Amphioxus the longitudinal epibranchial plexus, 
connecting the ventral part of the spinal nerves above the- 
gill-slits and supplyirg the latter with its branches, com- 
municates, according to HATSCHEK (1892, p. 144), with the 
medulla mainly by means of the third and, especially, the 
strong fourth segmental nerve (his fourth and fifth), we 
find in Petromyzon a similar horizontal nerve-stem connect- 
ing the epibranchial placodes and supplying the gill-slits 
with rami post- and praetrematici. The communication be- 
tween this collector, the ramus branchio-intestinalis vagi, 
and the brain in Petromyzon and other gill-breathing Craniates, 
however, is wholly established by the third (glossopharyng- 
eus) and, especially, by the fourth nerve, the vagus (cf. fig. 20), 
while the connections witn the spinal nerves following 
behind the vagus, so well developed still in Amphioxus, are 
represented in Pefromyzon by insignificant anastomoses only 
of these nerves with the ramus branchio-intestinalis of the 
vagus, as described by RANSOM and THOMPSON (1886, p. 422) 
and FüRBRINGER (1897). In front of the vagus, however, the 
segmental communications between the anterior cranial 
ganglia and the epibranchial placodes have been retained. 
Originally the epibranchial placodes of the 7 and the 9t® 
nerve also are united to those of the vagus by the horizon- 
tal plexus from which, however, they afterwards separate 
(KOLTZOFF, 1902, p. 531) Thus we may consider the vagus, 
with HATSCHEK (1892, p. 152, as a partially polymeric 
nerve that has collected, in its ramus branchio-intestinalis, 
