ELASMOBRANCH BRAIN ORGANIZATION 



133 



,AB C 

 dOD 28 



Figure 8 Dorsal view of the brain of the chimaera, Hydrolagus colliei. alll, anterior 

 lateral-line lobe; b, buccal ramus of anterior lateral-line nerve; dob, dorsal division of 

 olfactory bulb; il, inferior lobe of the hypothalamus; oe, olfactory epithelium (organ); ot, 

 optic tectum; p, deep ophthalmic or profundus nerve; plln, posterior lateral-line nerve; so, 

 superficial ophthalmic rami of anterior lateral-line and trigeminal nerves; th, telencephalic 

 hemisphere; tm, telencephalon medium; vob, ventral division of olfactory bulb; II, optic 

 nerve; V, mandibular and maxillary rami of trigeminal nerve; VII, facial nerve; VIII, 

 statoacoustic nerve; IX, glossopharyngeal nerve; X, vagal nerve. 



overlapped by the cerebellum and characterized by hypertrophy of the super- 

 ficial tectal zone, extensive migrated diencephalic nuclei, and hypertrophy of 

 the telencephalon (figures 4-6, 12-14). This brain pattern will be referred to 

 as the galeomorph pattern. 



Exceptions to these general trends do occur. Scyliorhinids have the com- 

 plex telencephalic and tectal development characteristic of the galeomorph 

 pattern, but their cerebellum is unconvoluted, like that of squalomorph 

 sharks. Heterodontus, the horned shark, is a problematic taxon. Its general 

 brain form suggests close affinity to squalomorph sharks, with which it has 

 frequently been grouped. However, Compagno (1973) believes Heterodontus 

 is closely related to the galeomorph orectolobids. Details of its neural 

 organization are lacking, so it is impossible to determine whether the 

 resemblance to squalomorph sharks is more than superficial. 



All batoids possess complex telencephalic and diencephalic organization 

 similar to that of galeomorph sharks. Cellular migration and thickening of the 

 telencephalic wall reduces the lateral ventricles to mere vestiges (compare 

 Figure 10 to Figure 15). Rajiforms and torpediniforms have a simple or 

 slightly convoluted cerebellum (Figures 11, 16), while the myliobatiforms 

 have an independently evolved large brain size and a complexly convoluted 

 cerebellum (Figures 1, 11, 17) whose asymmetry parallels that of carcharhinid 

 and sphyrnid sharks. 



