270 



ANATOMY OF VERTEBRATES. 



Lamprey. The second pair is niucli larger ; it passes out of the 

 neural canal, anterior to the first myocomma, and sends a branch, 

 170 c, fig. 170, upward and backward toward the front 



edge of that segment, which communicates with 

 the dorsal branches of several successive nerves of 

 its own side, like the liranch from the coml^ined 

 trifacial and vagal nerves, marked i in fig. 204 : 

 the main trunk of the second nerve curves 

 downward and backward, d, fig. 170, communi- 

 cating with the corresi)onding jiwcis of the suc- 

 ceeding nerves of its own side, to some way 

 beyond the vent, fig. 169, a s : this portion answers 

 to the branches 3 and 4 of the ' nervus lateralis ' 

 in fig. 204. From the princip)al function of the 

 second (conspicuous) pair of nerves in the Laneelet, 

 as a ' nerve of association,' it probably answers to 

 both the trigeminal and vagal, which in most 

 higher fishes combine to form the ' lateral nerve,' 

 with the same relations to the spinal nerves and 

 median fins as the nerves c and d, fig. 170, show 

 in the Laneelet. 



Costa ' describes and figures ' la macchla bruna 

 degli occhi ' (p. 14), ' I'opacita corrispondente 

 sopra e dietro degli occhi' (ib.), '^ and 'talvota i 

 gangli olfattori ' (Tab. i., fig. 2, r/y). Eetzius' re- 

 discovers the ocellus ; and KoUiker'' has more 

 particularly described the sub-terminal ciliated 

 depression, described as an ' olfactory sac,' and 

 indicated in the diagram, fig. 169, ol. According 

 to those observations, olfiictory and optic ner^c- 

 filaments may be inferred ; and the fore part of 

 the neural axis, including the trigemino-vagal 

 nerves, c b, fig. 169, will answer to the brain. 



The succeeding; nerves divide, soon after cmero-- 

 ing, into dorsal and ventral branches, as in higher 

 fishes, corresponding in number Avith the muscnlar 

 segments. The nerves consist of the primitive 

 cylindrical fibres. 



This is the most simple persistent condition of 

 the central organs of the nervous system known 



ln'iicculnliuit. 



XXX. 



( XXT, 



" Qiicri/, can tliis opako spot be an acoustic sac? 



' XNXII, 



