ORIGIN AND STRUCTURE OF THE HEAD 143 



as a rule a separation of the (visceral) motor part from 

 the ganglion in the form of the sympathetic ganglia. Thus 

 the four head ganglia contain two elements which are absent 

 in the spinal ganglia, 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 ElSlG 

 (1887) and HATSCHEK (1892, p. 151-'52), 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 supplying 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 with the spinal nerves following 

 behind the vagus, so well developed still in Amphioxus, are 

 represented in Petromyzon 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 FuRBRlNGER (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 9*^ 

 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,' 



