The lamprey is of particular interest in the origin of the ya- 

 gus since there is no indication of missing segments; each so- 

 mite which appears in the embryo is retained and served by 

 a segmental nerve. The vagus is not developed by the collec- 

 tion of segmental nerves into a single bundle but as a single 

 outgrowth which branches and extends back to all of the 

 branchial arches, behind the hyoid, and back into the body 

 cavity to the gut. The lateralis division grows outward in a 

 similar fashion. The development of the tenth cranial nerve 

 is thus like that of the fifth and seventh. The seventh and 

 tenth roots are joined by a connective outside the octic 

 capsule. 



Myxinid The brain of the myxinid is quite unlike that of 

 the lamprey (Figure 13-17). It lacks any chorioid plexus or 

 extensive membranous roofs. This lack is apparently due to 

 late changes in development and is not primitive. The pos- 

 terior part of the brain (myelencephalon) is large, while the 

 anterior three segments are relatively reduced. From front 

 to back there are paired olfactory bulbs, cephalic enlarge- 

 ments, diencephalic and mesencephalic lobes. The meten- 

 cephalon cannot be distinguished or is lacking. There is a 

 sweUing above the midline groove of the diencephalon 

 formed by the fused anterior and habenular commissures. 

 Behind these is a small epiphyseal evagination. Ventrally 



pineal body. 

 (moved to left) 



olfactory nerve (I) 

 olfactory bulb 



telencephalon 



fourth ventricle — 

 (membranous roof; 

 cut away) 



paraphysis in membranous roof 



jj_ superficial ophtholmic VII pituitary. 



%J> ^^ ^1,^% saccus vasculosus 



VI 

 buccal branch VII 



hyomondibular 

 branch of VII 



lateral-line lobe {restiform body) 

 VIII VIII 



superficial ophthalmic VII 

 cerebellum 



membranous roof 

 (cut away on left side) 



occipitospinal nerve 



A B 



Figure 13-13. Brain and cranial nerve roots of Hydrolagus. A, dorsal; B, lateral viev/. 



THE CONDUCTING AND INTEGRATING SYSTEM 



397 



