ELASMOBRANCH BRAIN ORGANIZATION 



137 



oe 



mob 



Figure 12 Dorsal view of the brain of a carcharhiniform shark, Mustelus canis, the 

 smooth dogfish, a, anterior division of the central nucleus; alll, anterior lateral-line lobe; 

 ar, anterior ramus of statoacoustic nerve; b, buccal ramus of anterior lateral-line nerve; c, 

 caudal division of the central nucleus; h, habenula; hy, hyomandibular trunk; il, inferior 

 lobe of the hypothalamus; lob, lateral division of the olfactory bulb; lp, lateral pallium; 

 m, mandibular ramus of the trigeminal nerve; mob, medial division of the olfactory bulb; 

 mx, maxillary ramus of the trigeminal nerve; o, otic ramus of the anterior lateral-line 

 nerve; oe, olfactory epithelium (organ); ot, optic tectum; pc, pallial commissure; pill, 

 posterior lateral-line lobe; plln, posterior lateral-line nerve; pr, posterior ramus of the 

 statoacoustic nerve; so, superficial ophthalmic ramus of the anterior lateral-line nerve; 

 so , superficial ophthalmic ramus of the trigeminal nerve; II, optic nerve; III, oculomotor 

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



The data in Figures 18 and 19 demonstrate clearly that elasmobranchs 

 possess large brains, comparable in size to many avian and mammalian 

 brains. In addition, the range of variation in brain size is approximately 

 the same for elasmobranchs as for other vertebrate classes. 



The extremely high brain: body ratios for elasmobranchs cannot be 

 attributed to the light weight of their cartilaginous skeletons relative to 

 bone. My analysis of Mustelus canis indicates that the skelton accounts 

 for 15% of its body weight. This figure is well within the range of skeletal 

 weight percentages reported for other vertebrates (Reynolds and Karlotski 

 1977). A minimum correction factor of 15-fold would be necessary for 

 elasmobranchs to be placed on the same principal axis as other anamniotic 

 vertebrates in a polygon. 



The elasmobranch coefficient of allometry is also very high relative to that 

 for other vertebrates. Based on data reported earlier (Northcutt 1977b), I 



